If Nether Spirit isn't the only creature card in your graveyard as your upkeep begins, its ability won't trigger. You can't take any actions during your turn before your upkeep begins.

If Nether Spirit isn't the only creature card in your graveyard as its ability resolves, it remains in your graveyard.

If you have two Nether Spirits in your graveyard, they stop each other from returning.