The main protagonist and the "16-year-old robot" to which the title refers, a state-of-the-art sentient gynoid automaton created by Dr. Noreen Wakeman five years prior to the series, though she was designed as a 16-year-old girl. Though XJ-9 was designed to be Earth's protector, armed to the teeth with a wide range of weapons, devices, and transformations, she desires to live the life of a normal teenager and often makes this desire quite apparent to her friends and creator.
Chivalrous Pervert: Towards Jenny. It may seem sometimes that he's only interested in Jenny's robot abilities and design, but in other episodes proven otherwise. For example, once Jenny wanted him to remove all her equipment, and he complied (although reluctantly) and spent hours on end removing all of it despite knowing how she would look afterwards. It much depends on the writer how Sheldon is.
Ineffectual Sympathetic Villain: Starts off as a huge threat, but becomes an easily dispatched threat later on. This really gets hammered home, though, when she eventually loses her dictatorship over Cluster Prime.
Even Mercs Have Standards: Even though she only does hero work for money and sees no problem with meditating while a Killer Rabbit destroys the town, she was clearly disgusted that her former teammates lazed around and actually bet against Jenny as she fought against a Giant Monstrous Frog.