There are 50 different companions to choose from in the game and each comes with their own uniques set of skills. Regardless of which companion you decide to equip their skills will all benefit Senia in some way.
Remember that all the companions you obtain as you progress through the game are going to be useful since the skills they possess will count even if the companion isn't currently equipped.
In our opinion the best companion to have equipped early in the game is Popo as doing so will allow you to get companion cards more easily and increases your maximum HP. Popo can be obtained as a gift when you make your first purchase.