Utaya was a border village in Almyra that was destroyed by giant monsters from Fodlan, leaving Kabru as the only survivor.
Kabru was rescued and taken in by Milsiril, a Nabatean who dislikes her own kind and has a habit of adopting human children. She teaches him everything he ever wants to know, except for why giant monsters only seem to exist in Fodlan despite the claim that they come from an animal having absorbed too much magic.
After much convincing of his very doting and over-protective adoptive mother (and a lot of very intense sword-training), Kabru makes his way to Garreg Mach to try and discover the truth of how/why giant monsters exist, and to join the Knights of Seiros specifically to fight them.
Kabru uses his connection to Milsiril (who is like a wayward cousin you still care about even if you never hear from them to Rhea) to gain access to the Church library and resources, but Rhea requires that he go through the Officer's Academy before he can join the Knights of Seiros.
Specific joined House I leave up to whatever makes sense for the thread.
time-skip
After the fall of Garreg Mach, Kabru wanders Fodlan with a small "mercenary band" (his canon adventuring party) primarily responding to reports of giant monsters.
They are not very good at fighting them and barely escape with their lives many times but they don't actually charge for it so...
When he hears people are gathering at Garreg Mach again, Kabru returns as well.
(frozen comment) fire emblem 3 houses au
background & pre-timeskip
time-skip