Simian

Simian is a character introduced in season 2 of Ben 10: Alien Force, an Arachnichimp(Spidermonkey's species) from Aranhaschimmia. He appeared in Birds of a Feather, wich was, so far, his only appearance.

History
Simian appeared for the first time in Birds of a Feather, where, coming on Earth, he met Ben(as Spidermonkey), Gwen and Kevin being attacked by a group of DNAliens. Mistaking Spidermonkey for a true Arachnichimp, Simion interfered in the fight and forced the DNAliens to leave, saving the group. When learning he had actually saved Ben 10 and his team, he decline their offer to join their team to fight DNAliens, stating he couldn't.

Later however, Ben surprised him stealing tech from a human lab. He helped him to escape as Goop, but asked for an explanation. Simian then replied he was the Prince of Arachscimia, and, to prove he was a valuable king, had to get and bring back on his planet a crystal from a base on Earth moon. Feeling empathy for him, Ben decided to help him, and convinced reluctant Gwen and Kevin to come too.

Arrived on moon, the group successfully passed through the base's defenses, but Ben started having doubts about Simian's motivations when the Arachnichimp asked him to attack Lu, the harmless guard of the base, and tried to kill him himself when Ben refused. It was eventually revealed that Simian had lied to Ben, and was actually a mercenary hired by a Highbreed to steal the crystal, who was the power source of a communication base needed by the Plumbers to protect Earth from the invasion. However, when bringing it back to his hirer, Simian realized too late that Ben had replacing the crystal inside the container by an earwax filled with worthless but powerful alien acid. Thinking this was a bad joke from Simian, the Hightbreed seemingly killed him off-screen with the acid.