There are many friendly NPCs during boss fights. You see, every time a boss teleports to a certain spot, or faces a certain direction, there needs to be a NPC at that position so that the game can tell the boss "Target that NPC and face in his direction".
Therefore, I don't think this has anything to do with a secret boss; the developers just forgot to make this NPC invisible to the players.
Here are some other examples of these NPCs that I specifically remember, but I'm sure there are lots more:
: Soa in Eternity Vault, the position where Soa teleports to when you jump down the platforms
- Saturation Fire
: Kephess in Denova, the area where he puts his AOE attack in
- Laser Focus
: Golden Fury in Makeb, he casts his red laser at this fountain