The Therapist is an NPC that comes to help Sims when they go into aspiration failure (Aspiration bar in total red). He examines the Sims and helps them get back a little of their Aspiration Bar, then disappears. The Therapist is just a "mirage" seen by Sims in aspiration failure and does not exist.
With cheats, it's possible to have a baby with the Therapist. Although the Therapist does not have very good genetics and most offspring have very large eyes right next to each other and gaunt faces. The shrink can only be seen by the Sim that he is taking care of, and the player can only see the shrink if the Sim the therapist is tending to is selected. But Sims in the same room will turn to the Sim being helped and do the 'You're Crazy!' sign. This could be a reference to The Sixth Sense where only the boy can see the therapist, Bruce Willis.
Making the Therapist selectable or adding him to a family will cause game corruption, as his character file is located in the objects.package file, and is the same for all neighborhoods. A much safer way to play as the Therapist is to create him in CAS with debugging cheats enabled. Since the Therapist has facial features that are highly customized, and eyebrows, glasses, and facial hair that can't be accessed in-game, the easiest way to play as the Therapist is to download an exact replica of him here.
Oddly, if the player Shift+Clicks on him while testingcheatsenabled is on, all the options that normally appear on normal Sims will appear, instead of the two used for Sims with incomplete data or universal Sims. Players should NOT click on any of these options, as it may cause game corruption.
When using SimPE, one can see that his last name is Therapist, making his technical name "Therapist Therapist". The Therapist cannot be seen by other Sims, other than the Sim he is healing. This is similar to what happens with a Sim and the Social Bunny.
The Therapist's hair was most likely black before it turned grey, seeing as if the player makes a Sim have a baby with him, they will usually have black hair. The Therapist's default hair color is identified in-game as black, and his genetics also seem to be very dominant.
It should be noted that he doesn't always have an aspiration when viewing SimPE. The Therapist's interests are different in each neighborhood. It should be noted that when viewing SimPE, he may not always have interests.
There is a small chance that the game may try using the Therapist in a walk-by. However this rarely happens, and if it does, he just shows up, stands there and eventually disappears. This is probably due to a glitch. According to SimPE, he is 3 days from being an elder, disregarding the fact that he doesn't age.