Samurott, as an Oshawott was obtained and given to Tsumugi by Professor Juniper. In Tepig, on the Loose!, he evolved into Dewott during a battle against Team Rocket and learned Water Pledge in the process.
In a future episode, he later evolved into his final form. As a Samurott, he is shown to maintain his loyalty to Tsumugi, he often protects her from bad people like Team Rocket.
In "What's In a Nickname?", he was officially nicknamed Sammy by Tsumugi which she briefly mentioned, and even nicknamed the other Pokemon she has.
In Season 3, it gains the ability to Synergy Evolve thanks to Tsumugi's determination, this allowed Mugi and Sammy to merge and become "Tsumugi-Samurott".