ToME: the Tales of Maj'Eyal

Yeeks are unlocked after beating Subject Z, yes?
I didn't have too much trouble beating Subject Z, but in that battle I may have hit the Yeek Wayist; though he says "You saved me?", he begins [or continues] battling me. Couldn't talk to him because that...killed him.
So I redid the battle, but now the Yeek Wayist disappears mid battle, seemingly always after a critical hit [log does not say he's killed], and never returns.
Am I guaranteed to unlock the Yeek if I defeat Subject Z, do not hit the Yeek Wayist, and if the Yeek doesn't "leave"? Or is it something else entirely?
Thanks,

If you damage the Yeek Wayist during the fight with Subject Z, he will turn hostile against you after Z is killed. If this happens, you have no other option then to kill him, and the unlock is lost, just as if he was killed by Z itself.

So, you need to be very quick with damage and very careful with AoE effects not to hit the Yeek.

Alternatively, you may get to the last floor on some lower level to have Subject Z generated as a lower-level monster, leave without triggering the fight, and come back when you're several levels higher. Subject Z will stay at the lower level at which it was generated, and you are pretty likely to beat him really quickly.