Or is it just there for theme and to eat up a unique item? And to be potentially sacrificed :') And does it count as one of the initial 11 allies?

It can't be sacrificed... says so on the card that you can print out on the scenario page

I found the ally really helpful - by pairing him with Harvey Walters, you make the wimpy professor a pretty good monster killer if you can get a good weapon into his hands.... We just played & he also drew "Enchant Weapon" as one of his spells. It was a great combination - he did not have to worry about sanity loss from horror checks because of his and the ally's special abilities - we got a tommy gun into his hands and with the enchant weapon, he didn;t have to worry about physical immunity.

Also, I think the new ally cannot be sacrificed.

When we played, we treated Calvin as an ally and immediately discarded one of the original 11 from the deck. (I think it was the professor ally, unfortunately for us.)

Calvin came in handy partnered with McGlen, who then would take one less sanity AND one less stamina damage. If his ability would stack with The Proffesor's own (and the prof's 7 sanity) then Prof. could be an excellent spell-caster (I've had success using him as such in a previous random game.)

Harvey has been a power house for us. His spell casting is still a little risky. But, if I understand the rules correctly, neither his ability nor the ally reduce the sanity cost of casting spells. They only reduce the sanity lost from monsters and encounters. Still, with all of his sanity and high lore he is a good spell caster none the less.

Ex-zactly. ^___^

(that was what I meant, although it is possible the first time I played Harvey I used his less-sanity damage ability incorrectly, or else was only casting 0 and 1 sanity cost spells.. I believe the latter to be the case, since my cohort has always been strict on the rules.)

yep, we put Calvin with Harvey, though I wanted the cutie for myself....