User blog comment:SoulRise99/HISHE: How Fairy Tail Should Have Ended/@comment-25262217-20170726230203/@comment-2601:183:8480:9F3B:B9CE:5FA6:FD32:90C6-20170727004730

Natsu was killed by Zeref and then Lucy brought him back to life.

No one knows what happened to Aquarius's key at all. Maybe summoning her is via Caster Type instead of Holder Type like the others.