Glimmerskin
The Glimmerskin, along with the ixitxachitl and the unbodied, was one of the craziest monsters that WotC ever tried to make playable in Dungeons & Dragons. Described in the Monster Manual II as "thrill-seeking parasites from the positive energy plane", Glimmerskins speak no languages, have no physical form, and cannot directly interact with objects in the material plane in any way, nor can they stay for more than a few hours before they have to go home. The only thing they can do while on the material plane is "bond" to a "host", granting the host increased combat prowess in exchange for... well, nothing. The Glimmerskin doesn't actually control the host during this time like the Inspired, nor merge with the host's consciousness like a Kalashtar. Instead, it offers suggestions like a telepathic backseat driver, much to the annoyance of the host. Glimmerskins do this because they like to watch the fighting, even if they can't directly participate in it.
Exactly why wotC thought these guys would make a good player race is unknown. However, for anyone who wants to try, their favored class is Fighter and their ECL is their class levels +15. Since they have 12 Outsider hit dice, we can derive a level adjustment of +3.