Mentor Spirits doesn't really have force at all. They are not really spirits 'in that sense' (they don't use powers, they don't materialize, they can't be banished etc.)
Mostly think of them as imaginary friends that only you see that guides you in the practices of magic also provide certain advantages and disadvantages that reflects it's mentor spirit archetype.
edit. also, welcome to the forum :-)
Thanks =)
But imaginary friends though? I mean one of them is the adversary, literally the devil.
And he is known in the lore to take a very active role in trying to cause misery and destroy all creation, so they ain't that imaginary.
Let me see if I get this right...
You are saying there is something there, to your right (or left) telling you to do bad things?

Might I suggest a doctor and a padded room??

But in all Seriousness, A Mentor Spirit is NOT a spirit in the conventional sense of the word. If 2 mages are standing side by side, but one is a rat Shaman, and the other a Hermetic mage, that Mage will never, EVER see, hear or even sense the presence of Rat even when directly talking to the Shaman...
To everyone else, the mentor is simply is non entity. Something they will never see, feel, hear, taste, touch, or smell, or sense. BUT to the shaman, the mentor is something that is VERY much real, and VERY much important - its the "source" of his magic! So those with Mentors treat them very seriously, and do their best to keep them happy....
Which in itself can lead to some... interesting issues that are best left solved at the playing table BEFORE a session begins.... (Some GMs and players have decidedly different
opinions on the personalities and ambitions of some Mentor Spirits...)