As you all know demons on Linked by fate... or Open Hearts... often give you quests.
And here some info about it:
~ one demon can give quest MAX 3 times in one day
~after completing quest you get CODE exp and random thing
but...
More quest you will do in row, better prize will w8 for you. Remember one thing, if you think you wont do quest DO NOT accept it, just reject. That way you wont loose number of quest completed in row. After 4 done in row you will start to get more rare titles and things (like small/medium/large runestone)
So next time your demon will give you quest, try your best to complete it! :3