Another disproportional 4-in-the-morning-get-out-of-my-skull drawing. I have a love/hate relationship with my imagination, FYI.

The idea is that these mages merge themselves to some astral spirit, which puts them both in this reality and the astral plane (whatever your favourite fantasy book calls it), and this is what gives them power. Risk factors include loss of sanity/self, and death! But it would seem the process only goes as far as the person wants it after the ritual.

The mage's spirit is called Sofia because I read that and it stuck.

The spirits start out as formless, and through the person's meditation become reflections of part of their psyche, could be their Shadow or as seen here, Anima. So they give the spirit a lot of themselves, resulting in it wanting more, and eventually trying to take over the person. At the moment, the spirit must be subdued and bound. The next step of the process is never revealed to the initiate until they reach it, but I think someone his spirit got a gist of it beforehand and was inconsolable. They had to go through with it anyway, but she never really struggled or tried to take him over.
The mage refuses to lose his humanity just as much as he refuses to "kill" Sofia. She is asexual herself, not affecting his tastes, and a mostly subconscious part of him - but has made him much kinder all the same.

Some will, after absorbing the spirit, loosen their ties to this world, not really feeding their bodies but living on all the same. Their power over matter is greater. The mage is not particularly tempted, but it does leave him a little bit of a loner "in the mage community".
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This is a big jumble of references.