Her roles
she does her partSince, I-Pin was introduce in the story, she had a part to play. While it may not be as major as the rest, her part is just as important. I-Pin is one of the ones that stays behind, so the others have someone to come back to. And that's just as important as going out to fight.
Daily Life
This is when she is first introduced. She often attacks Lambo for being a broccoli monster. And tried to asassinated Tsuna that one time. She even devolps a small crush on Hibari. (Can't blame her, he has great looking hair.) It's pretty much day to day life, meeting other mafia familes and getting in a few fights. Mostly with Lambo. And living at Tsuna's house. I-Pin and some others all took to calling Tsuna's mom, Mama.
Kokuyo Arc
I-Pin alongside Lambo protect Kyoko and Haru from the Twins. Lambo uses the ten year cannon on I-Pin and himself, making them more useful for the fight.
Varia Arc
I-Pin is not a Vongola Guardian, so her role was limited. She instead waits for the fighting to end. I-Pin also watches over Lambo with Mama (Sawada Nana) when he is in the hospital after the fight with Levi. (Because Levi is a total jerk who beats up 5 year olds in cute cow costumes.)