Loyal

Subverting all other loyalties in favor of a single person, organization, or cause. Taken to the extreme, this can be seen as fanaticism.

Vice: Higher loyalty to any other other person, organization, or cause in conflict.
Note: it is possible to have two loyalties (e.g., kingdom and religion), but when those two come into conflict, you have to choose - one is a virtue and the other becomes a vice.

See also:
Virtue house rules