You are right that leader is the wrong word for Tarvek (though it suits Gil to a T). He's not a natural follower though either, nor, I think, a lone wolf by inclination. So he wouldn't be liable to just follow the more charismatic Gil. He may be more of a... considered follower; if you convinced him you were worthwhile. That may be what the young Gil did?

Hee. I like the comparision between him and Boris, who is another favorite of mine (I'd love to see Boris with Mama Gkika, she's about the only one who could draw him out I think). They do both have that sense of hair-pulling responsibility. My feeling is however, that Tarvek is less of an accountant than Boris; more capable of pulling back and do strategy rather than just tactics.