The Piranha Bean actually looks similar to Petey Piranha except for being smaller.
In battle, the Piranha Bean mainly attacks by spitting a small bean-like object at Luigi. The bean object turns out to be none other than Prince Peasley who somehow turned small. Like most Piranha Plants in this game, the Piranha Bean can also spit fireballs at Luigi which, sometimes, can be tough to avoid.
Being a red Piranha Plant, the Piranha Bean is very weak to the Thunderhand techique so Luigi needs to stick with that for the entire battle (Mario isn't around so Luigi cannot use any Bros. Moves).