Elena/Dragary/Raenda/Midas

Elena
She's the first side quests master in this game which you will come across with at the first town which is Greenhill and she will hand out both side quests and main quests.

Dragary
He's one of the NPCs that has mission to give you side quests in Great Sandy area. When you see a "!" that's yellow that means that he has a side quest for you that you can pick and do. More on the quests area just klick on the side quests and you'll come to the page with the quests right away. But be aware if you level up way to much you can't back track and do the side quests! so be sure to do them all when you get them.

Raenda
She's the third side quests master which you can find in Maze Castle.

Midas
She's the fourth side quests master which you can find in Lost City.