Hydrogen

"Not to be condescending but... I did tell you it doesn't work that way... at the start..."

Invaluable as they may be for the Life Crew, when you first meet Hydrogen, chances are that they'll come off as rather persnickety. They can be awfully perfectionistic, idealistic to a fault, and rather too hung up on rules and social customs, maybe a bit judgy even. Usually almost comically serious for their childlike appearance, they're usually found staring into space, lost in thought. However, they have quite a temper too, either exploding outright, or quietly and near invisibly seething under the surface (which is arguably even worse as you'll still get burned if you approach).
Hydrogen's aforementioned importance for the Life Crew lies largely in their massive amount of knowledge and intuitive insight into the workings of the universe, them being the only element who remembers something "before Earth". Unfortunately, they usually find it difficult to express this, frequently speaking in a cryptic manner if at all. Despite their real age and great knowledge, they're still physically and mentally a child, and can get overwhelmed and act out very easily. Still, they often manage to put their knowledge to good use; for instance, they are the first element to date that has contacted the antimatter parallel universe, even managing to hold a short conversation with their counterpart... and more, however, they've been sworn to secrecy about that.

Random facts

  Hydrogen is the shortest element, standing at about 135 cm (4'5"). They're also the lightest by weight.
  They have the longest hair length, though - while they keep it tied up in braids usually, it reaches to the floor when down. They refuse to cut their hair unless absolutely necessary, for reasons.
  They have a whole lot of little rituals, such as wishing on stars, burning candles at certain times and the like. These are VERY IMPORTANT and Hydrogen will be upset if something gets in the way.
  They're very attached to Carbon, arguably a bit too much? The two are in a permanent covalent bond, currently the only example of this bond type that isn't romantic but friendly/familial in nature. This helps Carbon understand what Hydrogen is cryptically trying to convey, resulting in him becoming a sort of mouthpiece for them; often when Hydrogen talks even among the Life Crew, it's indirectly through Carbon. He's somewhat of a fatherly figure to them. No, Hydrogen won't go to bed at 8pm, DAD
  They have a little kid crush on Helium. She's completely oblivious to it.