You could use the skeletons by overriding their model, but your custom npc would be, in that case, only something that runs at the player (if they can find the player on their navmesh, generated from both respawn rooms) and melees them.
Alternatively if you wanted to do what the training modes do and spawn actual TF2 bots into maps (these do take up player slots though!) you can try looking at the decompiled versions here:
http://tf2maps.net/threads/valve-maps-decompiled.5952/ (in misc at the bottom). NPCs that shoot at the player and run around and take cover would be a lot more skillful to play against, but still overall would be bots.
You could maybe find a way to use the
attributes entity to add in a custom texture to the bots as well, but in that case (because the texture would be hidden by cosmetics these days) you might as well just colorise the player with AddOutput 'rendermode 1' (color) and then setting their 'rendercolor' to an rgb value. - Just to give the bots a different feeling, like totally green guys.