As a trainer like you, all you were doing was minding your own business. Just entertaining yourself since there is nothing 'special' happening today. However, your Greninja is the opposite, it is just outside of your backyard. Training with its own skills in order to get stronger for future battles. At least that is something Greninja can keep itself motivate, beside having to sit a room forever till the evening.

But that changed when your mother's footsteps are heard as she opens the door without any knocking. She tells you that you are going to your cousins/friends party, but she doesn't take no as an answer. She WANTS you to be at the party without any complaints! Even though you hate parties with reasons. Your mother gestures you to get ready quickly and take your pokémon with you, oh this is going to be a pain.

as you arrived at the party (which takes place outside), you could already hear the music blasting to your ears while children are either screaming or just talking loudly. Food can be smell from the table and so many people are in the party, especially the ones you don't even recognize OR remember.

Suddenly your pokéball starts to shake and Greninja escaped out of your pokéball, the height is just them being a bit taller than you,it flinched due to the loudness around the party. It is very confused on why they were forced to be at a party that the two didn't signed up for.

Greninja would look around and its eyes widen a bit, realizing that there is kids. Kids who are glancing at it with curiosity, some even approaching to Greninja. Oh nah this is the worst timing for it to come out..

"... Nin.. Ninja, gren." Greninja silently muttered, staying beside you.