Nemo is the captain of the Nautilus, a submarine manned by individuals looking to stop the Neo-Atlanteans at all costs. He is a mysterious man with a dark past who acts cold when he first meets Jean and Nadia, but later warms up to Jean's outgoing cheerfulness and enthusiasm. He becomes a father-figure and mentor to both Jean and (most especially) Nadia.
It is eventually revealed that he is the biological father of Nadia and rightful ruler of Atlantis, a the latter being a position he rejected in favour of peacefully roaming the oceans and living amongst the humans. He also has a secret relationship with Electra, who he impregnated and eventually gives birth to Nadia's half-sibling.