It may contain SPOILERS.
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Romeo is Bianchi's ex-boyfriend who died due to food poisoning most likely from Bianchi. Before Bianchi broke up with him, they had a horrible relationship, and every time Bianchi thinks of him, she gets upset.
He looks exactly like Adult Lambo and because of that fact, Tsuna tries to use Adult Lambo to distract Bianchi, such as when they were making Valentine's Day chocolates. He looks very much like Adult Lambo, except for a few minor differences; such as, he has blue eyes whereas Lambo has green eyes. Romeo also has his left eye closed, whereas Lambo has his right eye closed, and finally Adult Lambo has a tattoo beneath his eye which Romeo doesn't.
During Tsuna's test of courage, his ghost shows up and pretends to be Lambo. He leads Tsuna to a magic gate, after which he reveals his real aim - to take revenge from Bianchi, and hurt everyone that has ever associated with her. When the real Lambo came, he dragged both Tsuna and Adult Lambo to the gate, but they were saved by Bianchi, who defeated Romeo with her Poison Cooking.