Maximillion B. VonMitternacht would be more than happy to have fulfilled a Mentorship role, in terms of helping her manifest her powers and abilities. He is no adept at magic (due to magic in his Home Plane, despite being immensely powerful, is also a complex science), but he knows a few spells and charms and enchantments that are low-tier, but often handy in situations that might require it.

Rahvin Ardeaux would help, but he is extremely selfish, sadistic, and he makes no attempt to hide it, so if your character is more low-key, and, well...NICE and GENTLE, Rahvin might not be a great fit (but would be willing to have her be aware of him, to some degree).

Luther King Grew up 'in the hood', and his core belief system revolves around survival, and - to a large degree - lawlessness. As a result of being caught in the crossfire of a gunfight between gangs, Luther was hospitalized in critical shape, and submitted to an experimental procedure that embedded cybernetics into his body. He might be a source of worldly street wisdom, and even an ally, if Reimu can impress him.

Reidh Thorljottson Rowdy, rebellious young man, might be a thrill to be around, if you like a 'bad boy' with a heart of gold.

Exodus Soulcry is a man of phenomenal arcane means (although a specialty with Necromancy) who would teach her whatever she needs to know, if she finds his Temple, and rouses him out of his depressive mood, which he is almost guaranteed to be in (the depressive mood, AND in reference to the Temple). Unless in a particularly stern mood (or setting, like Family Meetings), he just generally comes off as a scruffy, middle aged man with a major depressive disorder, and who seems emotionally detached from the world around him.