Often enough when players have built out their character back story and create the actual character, they believe that they are done. But there are many ways that you can loop in the mechanics of your character to enhance the overall theme and characteristics.
Ability Scores
Perhaps while building out your character, you have certain ideas about the types of traits that they have and you want your character to mechanically represent that.
Now Ability scores are often set to min-max for your character’s combat and skill style, but that approach is so cookie cutter.
Perhaps your Bard who is naturally Charismatic is also highly Learned and has a lot of historical knowledge gained during her travels. Perhaps instead of putting your second highest Ability score in something like DEX to improve your AC, you instead put it in INT for that knowledge.
Subclass / Archetype
The subclass or archetype chosen for your character is a high mark in what type of person or what type of characteristics your character has. All though through a shell of wanting to fight things as a fighter, the type of subclass you choose to the fighter can add a whole new set of characteristics and narrative points for your character.
Lean into your archetype and make that a core part of your character’s identity. It is already the core part of your character mechanically.
Example:
As a Horizon Walker Hunter, I and mistrusting of open spaces, and when out and about in towns unfamiliar, I err towards alleyways and side street to be cautious and typically voice to my companions to keep their eyes peeled for threats. (Even if it's just a podunk town)
Features, Spells, and Feats
Perhaps you already have a certain identity or type of character in mind. They have a backstory that made them who they are today. Choose the features, spells and feats that would appropriately match that character.
Often enough, we build a backstory, but then when playing out combat or encounters, we use what is most powerful or useful from a mechanical standpoint.
“I am a sea sorcerer born of ships and practiced my magic on a pirate ship!”
Player: I cast firestorm.
Everyone else: ….
Interesting example:
Character: Jeremiah Spohlberg
Class: Fighter
Subclass: Cavalier
STR: 16 DEX: 8 CON: 14 INT: 12 WIS: 7 CHA: 15
I put the highest stat in STR because Jeremiah is meant to be the frontline for the party. So he needs to be able to hit hard. However, I put CHA as the second highest stat, because he comes from a noble background and is able to communicate with others from his upbringing. I put WIS as his lowest stat because Jeremiah is a young nobleman and isn't learned of the world and realistic decision making.
He is a Cavalier, so he rushes forth to aid others in battle. Because of this, he has a near inseparable relationship with his mount, it being his closest friend and partner. Because of this combination, I take the Charger Feat to allow his to rush into battle, displace his enemies, and it also provides an avenue for him to do it during less opportune times as a low WIS character.
