Guardians and Charges

The concept of the Guardian and Charge arose before humans were inducted into the UTO as a theory for successfully integrating the species. The first pairs were made up of soldiers that met and became rather close during the attempted rynar conquest of Earth.
The practical idea behind a pairing is that a nonhuman guardian, will take on the commitment to helping a human charge function in places that are inadequate for supporting humans. Such duties can be summed up as carrying them to locations and offering their hands as a place to stay in nonhuman crowds.
The psychological purpose behind guardians, however, was where the concept made its legacy. During human integration, guardians were critical for adapting the first humans to life with nonhumans. Naturally, many humans at the beginning were intimidated, afraid of nonhumans. The concept of a guardian offered a single nonhuman for a human to adapt to. The nature of what they are there for, a 'guardian,' helped to associate the nonhuman in the pair as a protector, not something to be afraid of. It was critical early on to assuage human fears, and establish positive experiences among humans.
In short time, it became readily apparent that pairs consistently lead to rapidly developing relationships, with both becoming exceptionally close to each other in a short time, which was as good an effect anyone could have hoped for in integrating humans. Within weeks, many pairs reached a point of feeling a strong emotional connection to their partner, and even before this, many pairs already became platonic.
About 84% of modern guardian and charge pairs lead to a physically affectionate relationship, though the degree to which this may be indulged varies wildly. Such affections may be as simple as mutual rubbing, scratching, petting and so on, to more aggressive physical interaction such as predator/prey games and oral activities.
The practical need for the pairs greatly diminishes to nearly nothing once a human has their mech and facilities adequately support them. But, despite this, many humans end up preferring their guardians to their mechs, and their guardians typically more than happy to take on the unnecessary duties. A mutual desire for each other's physical closeness almost always shows itself in the pairs.
Historically, the pairs became the source of most of the earliest human/nonhuman romantic relationships, and continue to be a regular source of such relationships.

Guardian Training

Guardians are selected on a volunteer basis followed up by a screening process. Screening involves background checks for any criminal history or mental instability, then an interview with a guardian trainer and a live questionnaire. There are no standardized questions in these interviews, with the intent being to prevent anyone from preparing for them. As such, they are normally morality questions and unlikely scenarios made up by the guardian trainer. For example: "What do you do if your charge tries to attack you?"
Such answers don't necessarily have a correct answer, and are more of a test to see how much a guardian will try to connect to the human's perspective of a situation, but also mind their own position, as a guardian nearly always has a superior perspective of the world around them. For example, a theoretical situation of a charge being terrified in a crowd. One potential guardian may describe covering their charge with both hands and trying to get out as soon as possible. Another may try to find a place to talk to their charge. The subtle difference between the two is the former takes initiative to try to physically remove their charge from the problem, where the other focuses more on their charge and not the environment that might be causing it. The latter isn't the 'wrong' answer, but it would depend on how the trainer judges the potential guardian's reasoning.
Once a guardian makes it through screening, they begin training, and are taught the following subjects:

• Hyper sensitive handling for the sake of being able to handle any human, including one that may be injured or the exceptionally young such as an infant.
• Sensitivity training to reign in certain behaviors such as compulsions to stroke or pet, fawn over cuteness, or cutesy names. The importance of this training greatly diminished once experience with humans became a standard part of every citizen's alien culture exposure education.
• Techniques for comforting their charges, such as offering a thumb for the human to hold onto, sounds they might find comfort in such as heartbeats or purring, and how to discover what a specific charge may like by encouraging open communication and careful experimentation.
• Simulated scenarios, such as aiding an injured human.
• Human emotional cues.
• Techniques for avoiding intimidating a human, such as toothless smiling and small posturing. It is important to note that being overly careful in this way has become offensive to many humans that are experienced with nonhumans, and is seen as condescending. It is mostly reserved for humans that may have never met a nonhuman before.
• Basic medical care for a human, including how to non-fatally administer CPR.

Guardian Duties

Guardians have an array of duties, and not all of them are just for their assigned charge.

• Offer carrying assistance to their charges.
• Protecting their charge from physical elements such as crowds.
• Offer emotional support to a charge in need.
• Be open to any instances where taking on multiple temporary charges is needed, such as being a designated, sober guardian during drinking nights with friends.
• Be ready to take temporary guardianship of a human that might need their assistance.
• Be prepared to offer assistance in human-centric emergencies, such as accidents, health crisis, or disasters.

Pairing

The method of pairing guardians and charges is dependent on the availability of guardians. While every pairing tries to create the ideal matches, this is not always possible. Simple communal living pairings have no obligations to be accepted, nor the pair maintained in the event the pair proves incompatible. Pairs within organizations such as schools, the military, or businesses may have their own protocols on pairings, depending on availability of guardians, and can require a guardian and charge to accept a pairing until it otherwise shows to be ineffective.
The ideal situation is a surplus of guardians. In this instance, a human is suggested several potential guardians based on psychological profiles and interests. If time is available, they may meet in simulation environments. If time is short, voice and video calls are the next option. The guardian and the charge must both consent to the pairing. If a charge is rejected by or rejects all of their potential guardians within some type of organization or company, the closest profile match may be required to pair, or offer some type of bonus or benefit in the hopes someone accepts. More often, though, the suitability of the charge being a part of the organization or business is brought into question.
If it's meant for roommates in a location with adequate human housing options and the human is rejected by their potential guardians, it will be up to the human to offer something, such as a greater portion of rent. If human housing options are limited or even nonexistent, then it will depend on the availability of foster guardians, most often lupari, who specialize in difficult charges. These foster pairs may grow into a longer term pair, but if the human manages to woefully offend the foster guardian before a suitable pairing can be made, then they will likely not be able to live in this location. Foster pairs are very rare, as by this point the human usually realizes they're not charge material. A human that rejected all of their potential guardians will not be offered any more and cannot be foster paired.
If guardians are limited, the closest profile match is presented, with simulation meeting or voice/video calls until a pair is formed. In case of all rejections, the same process as before applies.
In the instance of exceptionally limited guardians, such as a single guardian on a small colony and the presence of the human serves a critical role, they will be paired regardless of profiles. If the pair proves difficult, it will depend on the protocols of whatever organization they are a part of. If charges outnumber guardians, then a guardian may have multiple charges. Most pairings are not critical, as mechs make human independence in nonhuman areas easy, even when infrastructure is limited for humans. Because of the exceptional reliability of mainline model mechs even during extended use some humans may live mostly with their mechs, using their machine body more than their own and will never need to be paired. Humans that choose to be charges almost never have trouble being accepted by a guardian. The problems mostly arise in required pairings, such as micro-mechanical technicians, dual-pilot ships, or other professions that utilize the small size of humans.
The military requires pairings among its combat roles, as well as mandating regular contact outside of the machines with their guardians and others in their units. This correlates to a significant increase in morale and performance for the whole unit. The accepted idea is it helps nonhumans associate even the largest war mechs with a human they've physically held, and the accompanying protective instincts that come with it. Ideally, this will minimize over reliance on the machine, which often leads to destruction of the mech and death of the pilot. On the human's side, it nurtures a reverse-guardian instinct toward their squadmates when inside of a mech, helping them to maintain their nerves as the vanguard of most units.

Cultural Phenomenon

Over time, the actual need for guardians greatly diminished, as every nonhuman citizen had been taught to be able to act as a temporary guardian where needed and infrastructure was built to handle humans as a standard on hub worlds. However, tthe existence of such relationships went up in numbers, rather than diminished. Having a guardian became a staple of human life among nonhumans, and many wished to have a guardian despite not needing one.
The concept of the guardian and the charge had been heavily romanticized over the ensuring decades after integration as a relationship of intimate closeness where both in the pair find in each other a companionship that cannot be found in regular, same size relationships. It was the subject of many relationship dramas, war films, books, and so on.
There are always new guardians being trained and humans seeking to be paired with them, even if it is unnecessary. Many times, it is a roommate type arrangement, where a human simply wants to live with a nonhuman, and vice versa, with both of them desiring the relationship that spending a lot of time with another of opposing size forms.
In short, guardians and charges began as a practical relationship, but as that practical importance faded, the pair had become one of cultural tradition and importance, with many humans seeing having a guardian as a rite of passage into alien worlds, and nonhumans viewing having a charge as a mark of honor and integrity, with both desiring the unique friendship that a pairing brings.