First HP quest, the easy one. Go in tree, find Drizzle and say "Dri, replacement"

He will tell you the name of the new duty guard.

Once you did get the name of the new duty guard, climb down and right to your north-west there is a guard house, go inside and tell the guard in there "Name, replacement is X" where X is the name Drizzle did give you.

And your HP just increased by 100 !

 
Note : You will loose those 100 HP each time you leave Cobrahn, but get them back when you return to Cobrahn.