I am a newbie to this game, have played it about 5 days already and i have faced such situation.
Before that, as i summoned the partner to help, the partner was losing health as he or she was hit by the monsters, not otherwise.
But starting from yesterday, as i have unlocked Mikami (or how he was called ,that merchant), all partners started to lose health even just by standing. I can summon anyone, stand in the area for 10 minutes and half of partner's health would be gone! Without any reason! Without even being hit by anyone or anything!
And i want to know, is it NORMAL? Or it is some kind of glitch?
If it is, how to solve it? I mean, kinda wanna play with partners, but if they keep loosing health just by "appearing and standing for some times" that kinda...erm...

Yep, it's normal for the partner health to decrease after a while of summon.
Merchant, Miroku.
Their health will also recharge if u un-summon them.

Oh, so that is the case. Thank you! Now i am calm knowing it.