Tribal Bob was once a human.
We know that Peter once became a Manji, though he was cured, there is nothing saying it never happened before. All the other Manji have animal-like masks, but Bob's looks vaguely human-like, like Peter's did. Bob was once a player who got stuck in Jumanji, and became a Manji to survive. Unlike Peter, he didn't have anyone close to bring him back to his human form. Tribal Bob was once a girl.
We don't even know if "he" ever gave a name to the main characters, they just went with "Tribal Bob", and if Peter's near-permanent transformation is any indication, it is likely that Tribal Bob forgot "his" name anyway, in case "he" was a human once. Bob also happens to be smaller, with bigger eyes and more delicate features than other Manjis, and with long, flowing flocks of blonde hair on each side of "his" head. In other words, if Bob didn't already exist as a character, "he" would probably be most artists's idea of how a female Manji should look like. Squint was a real person in his first episode
While Squint was a jerk in his debut episode and lied to the main characters in order to get his revenge, it was a very personal goal and at least somewhat sympathetic. In successive episodes, he becomes an outright villain with generic goals and betrays others at the drop of a hat. Perhaps this Motive Decay was because he really was a person trapped in Jumanji who wanted revenge against the monster that had disfigured him, but once he died taking it down, Jumanji brought him back as one of its characters. The basic elements of Jumanji are regularly reset
The character pages mention some Negative Continuity for villains who seem to die or locations that seem to be destroyed in early appearances, only for them to appear like nothing had happened later on. Jumanji is a game, so it's entirely possible that what's happening is simply that it respawns certain elements when they have been destroyed. Like NPCs in most games, the characters themselves may not even be aware of this, as Trader Slick and Ibsen especially seem to be concerned when their assets are threatened, despite it never sticking. Jumanji may have some exceptions or alternate methods to accomplish this, however, as it apparently let the Sorceress of Jumanji stay defeated, and it once tried to have Peter replace Van Pelt to keep him around. The Running Gag of Slick having a different middle name every time isn't just a joke
Related to the above theory, perhaps each time Slick has a different name, it's a sign he has respawned and the game generated a new name for him, with such treatment being unique to him as part of his role as a duplicitous character. The Stalker isn't 'evil'
The Stalker is dangerous and powerful, being the physical embodiment of Jumanji, but not evil. It merely wanted the dice back, not to hurt Alan. If he'd turned over the dice it would have just walked away - it didn't want him to have the dice because they were a Game-Breaker and the game couldn't be played without them. Later, when Peter threatened the game, it was not specifically angry at him, but wanted to remind him of how powerful it was - they'd thought they'd destroyed it before, but it came back. Von Richter was once a real soldier.
Ludwig mentions coming to Jumanji. If he doesn’t come from another war-themed game, then he was probably a real person once. He could have been a decorated soldier in the German military who found Jumanji, seeing as how others have made it to the jungle without the board game, and saw it as a challenge of his hunting skills. Eventually he became another part of the game including his own compound to hunt on as he pleases.