Bem was traveling aboard the Enterprise as an independent observer during a series of exploratory and contact missions. He spent the first six missions in his quarters, until stardate 7403.6, when he finally emerged to accompany and observe the contact party that was preparing to investigate the aborigine lifeforms recently discovered on Delta Theta III.
Bem took advantage of the opportunity to observe not only the Delta Theta III aborigines but also the crew of the Enterprise. To this end, Bem surreptitiously deprived members of the landing party, on the planet, of their communicators and phasers in an attempt to force them to deal with the situation without technological help.
According to Bem, James T. Kirk had been identified by Starfleet as its best Captain and so he was chosen as the observation subject. Despite this, Bem felt Kirk's actions belied Starfleet's assessment, in particular his inability to rescue Bem from the aborigines. However, Bem was forced to reverse this judgment, to the point of considering disassembling himself, when his own actions jeopardized himself, the Enterprise crew, and the aborigines.
As a Pandronian, Bem was a colony creature. He referred to himself as "this one" rather than the expected "I," and addressed members of the Enterprise crew with their names, followed by their ranks, as in "Kirk Captain." (TAS: "Bem")
The character of Bem was originally intended to be featured in the third season of Star Trek: The Original Series. Writer David Gerrold named the alien "Bem," an oft-used acronym for "Bug Eyed Monster," simply because he reckoned it would be fun to have a true bug-eyed monster on Star Trek, although the character was never intended to be one and Gerrold never attained his wish, in regards to the original series. (TAS: "Bem" audio commentary)
In both of two early story outlines for the episode "Bem" (as proposed for that series), the character held the rank of lieutenant. He was a specialist in subspace mechanics and astrotechnology in the first outline, and a psycho-sociologist in the second, which also gave his full name as "Hari Bem". (The Trek 25th Anniversary Celebration)
David Gerrold deliberately tried to give Bem a unique personality, later saying of the character, "It was the opportunity to create a different kind of alien mindset. That he really did think differently than our original characters. And so he was puzzle." Additionally, Gerrold enthusiastically referred to this uniqueness as "one of the nice things about Bem," and described the character in general as "a kind of interesting alien." (TAS: "Bem" audio commentary)