Flower Fairy

[Limita]

is it worth leveling up to level 100 your flower fairy at the start of the game?

[Synthar]

no

[BlackInk]

From my experience, no. The flower fairy will either use a party slot and if you leave it with default skill, get you flowers, or slowly regenerate weeds and flowers.
You are better off using a loop action to get those, and when entering a dungeon, use a combat oriented ally

[Slartibartfast]

I'm no expert, but I would say in the early stages its not worth spending the seeds - the only benefit is an increase in Luck multiplier. The Flower Fairy is useful, but only for the flower regeneration in the early game, and once that is full swap it out for something else.

[Deathman.Kenshi]

Once you get to "final" boss there is oportunity to power grind all rare summons, you just need them unlocked and you can then grind theirs research super easy there. Also you want auto level slot for them and you will get that later in 1st run.