So I have been playing this game on and off for a couple weeks. My brother played this game for a while and I decided to play it too. He made a new character to play with me and we decided to adventure together as a group however only 15% of the quests We pick up are together. It seems like most of the time he gets quests I dont or vice versa.

Do each character/class get their own side quests and quest lines? I am playing a Sorceress and he plays a Shai. We've been at this for a while and I am getting frustrated over not being able to play quests together.

The quests are a mess in this game, and they ain't designed to be played as a group.

If you pick the same crosroad quests, you should have the same main quests tho, and only that questline is important.

There are actually very few quests in the game that are class-specific.

Some quests are per-character, and others are per-family, and many quests require prerequisite quests to be completed.  I'd guess that you're not seeing the same quests because he has completed some of the per-family quests or prerequisites and so is ahead of you on those.  My wife and I play together majority of the time, and occasionally when we start a new character for Seasons, we'll come across quests that only I see or vice versa.  It always turns out it's because one of us had started a quest chain on a different character and got ahead.

You can check a site like BDOCodex to see if the missing quest is part of a quest chain or has prerequisites.  Then you can find the start of the quest chain and do those quests until you catch up to your brother.

I've quested a lot with friends and most of the time it works fine, provided both/all players have the same types of quests visible. 

But if it happened that we found a quest one or more of us can't see, we'd probably check BDOCodex to see where the problem is.

You will play through questline up to 49 and after you hit 50 you will never see each other again. This game is mostly about grinding alone. I would even go so far to say that 98% of BDO is made to be played solo. This game punishes group play.

There are very few quests in the game that are specific.

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I get 2016 vibes. Welcome to BDO! Glad to have you here

You would be correct about this because not a lot about the grind for silver on your own without profit loss has changed about this game. When it does or if it ever changes in the next five years BDO will not be the game that you even know today. Until then, I stand by my statement 2. % of BDO's grind content is group friendly.