Dexter (デクスター) is a fictional boyfriend that Amy Rose fabricated in Sonic Chronicles: The Dark Brotherhood, to make Sonic jealous. Amy doesn't go into detail about him, and almost nothing is known about him. Depending on the choices of the player, Amy might admit to Sonic that he isn't real.
Sonic does actually have the chance to talk to her about Dexter almost every time he talks to her to make her mad or uncomfortable. Eventually in Chapter 5 when Amy says that she doesn't want him to get hurt he can tease her by asking, "Does your boyfriend know that you worry so much about me?" and she would angrily reply, "Why do you care so much about Dexter? It seems like he's your boyfriend not mine!".