Monsters, Villains, and Other NPCs (CPsiH)

Most of the common-sense rules that apply to regular monsters, villains, and NPCs apply equally to psionic figures. It never hurts to review, however, so some pointers follow.
Psionic NPCs make excellent major villains. But like any powerful, single entity - a vampire, a dragon, a lich, a beholder - they are much less effective in isolation than they are at the center of a web of minions. For this reason, most psionic villains will have numerous slaves, followers, mercenaries, and other vassals at their beck and call. Some of these may be minor psionicists in their own right. As well as acting as the master's eyes, ears, and gofers, they serve to wear down attackers before the final confrontation (or they buy time for the psionicist to escape if the opposition is overwhelming).
Psionic monsters, on the other hand, tend to be solitary creatures. Many of them hunt by keying in on mental activity, or even on purely psionic activity (they detect and track psionic activity the way a hound picks up the scent of a fox and mercilessly hunts it down). A crowd of thinking beings tends to create interference (i.e., "jam the airwaves"), making it more difficult to notice an actual psionicist (or choice victim) who wanders into range.
Because they operate alone or in small groups, psionic creatures tend to strike quickly, inflict as much damage as possible, and then melt away before the victims can organize a counterattack. An excellent tactic, for example, is to teleport into a group, grab someone at random, and teleport away immediately. Suddenly, that one person can be dealt with alone, rather than in the midst of his comrades. Tactics like this can cause far more fear and destruction than a glance at the creature's statistics might inspire.
Even when they're friendly, NPCs with psionic powers tend to keep their those powers to themselves. Secrecy is always the best policy. Unless there is a good social or employment reason for it, never reveal that an NPC is a psionicist; let the players figure it out for themselves.