Source - https://fairytail.fandom.com/wiki/Aquarius/Image_Gallery?file=Aquarius_gives_her_key_to_Lucy.png
In the picture it's just after Wendy has transformed into Aquarius and received her celestial key to give to Lucy.
Lucy was doing a mission and had to fight a powerful enemy wizard and she summoned most of her celestial spirit's to fight but they couldn't damage him and Aquarius was the last to be summoned, Lucy asked Aquarius to attack the enemy and she aimed a powerful blast of water at him but the enemy quickly dodged and attacked with a powerful electric spell which was really hurting Aquarius, Lucy tried to help Aquarius but she was feeling tired from all the summoning she had done in the battle then the enemy hit Lucy with a spell knocking her back and keep hitting Aquarius with electric spells.
Lucy could do nothing but watch as Aquarius looked at Lucy and said "I'm really sorry Lucy this might be it for me" and she vanished and Aquarius's key shattered "No this cant be happening I'm so sorry Aquarius I didn't know this would happen" said a crying Lucy and the enemy wizard just ran away while Lucy was crying.
Lucy returned to the Fairy Tail guild looking drained and she collapsed which got the attention of the other members of the guild after she recovered they were asking what happened and Lucy calmly explained what happend during her fight and Natsu hearing what happened he left in a hurry to find the powerful wizard and beat him to a pulp for making his friend unhappy and Grey and Erza followed after Natsu.
After Lucy had something to eat she went home and went straight to bed after what she experienced that day, Wendy who heard what Lucy said was wondering how to cheer Lucy up and heard the Celestial Spirit King talking in her head and he asked "Do you want to help Lucy?" Wendy was surprised that the Celestial Spirit king was talking to her and she replied "Yes I absolutely want to help in any way I can" "You are a very kind friend indeed" replied the Celestial Spirit King.
Just then both Lucy and Wendy were teleported to the Celestial Spirit World which surprised both girls and they saw all the Celestial Spirits there happy to see them but sad at the same time because of the death of Aquarius, "what have you summoned us for Celestial Spirit King?" asked Lucy "well Wendy here has agreed to help you as a true friend" replied the Celestial Spirit King then Lucy looked at Wendy who smiled and nodded.
The Celestial Spirit King used his magic and a beam of light shined over Wendy and Lucy covered her eyes and after a few seconds the beam of light went away and Lucy uncovered her eyes and saw Aquarius with dark blue hair in front of her and asked "Is that you Aquarius?" and she replied "No it's me Wendy it looks like I've been transformed into Aquarius".
"It is done" said the Celestial Spirit King "you are now the new Aquarius", "when I said I wanted to help Lucy I didn't have this in mind" said a worried Wendy checking out her new body, then a key appeared in Wendy's hand "this might not be what you had in mind but it means a lot if a friend of Lucy's was there to help when she needs it plus we need someone else to take Aquarius's place since she's is no longer with us" said the Celestial Spirit King.
"I won't let you down sir" replied Wendy and she gave her new key to Lucy "I guess I'm one of your celestial spirits now and I hope to be very helpful to you in battle and don't worry I will visit the Fairy Tail guild when I can but I will have to get used to my new powers so I'll be training" said Wendy "thank you for doing this Wendy it means a lot" replied Lucy and they hugged and then the Celestial Spirit King teleported Lucy back to her room.
The next day Lucy went to the guild and couldn't find Natsu,Grey or Erza so Lucy asked Mirajane if they have returned yet and she replied that they are still out there trying to beat the wizard that you struggled with and Lucy replied that she was going to go and find them and left the guild.
Lucy returned to the area where she faced the wizard and saw Natsu,Grey and Erza on the floor and she ran up to them and asked what happened "We saw the wizard that you described but when we went to fight him it was like our attacks didnt have much of an effect" replied Natsu " And when all three of us attacked at the same time we suddenly got attacked out of nowhere" added Erza.
"Well now that I'm here let's defeat him together and they hurried to find the enemy wizard, after a little while they found him "you lot don't give up do you?" he asks "we're members of Fairy Tail we don't quit till we get the job done" replied Natsu and he charged at the wizard and this time Lucy was going to pay attention to the enemy and she saw what looked like a barrier appeared for a second which blocked Natsu's punch.
Natsu keep punching and used his fire breath magic but they keep getting blocked "What gives why can't I hit you?" "since I know your guild is really powerful I gave myself major defence for when fighting against your guild" the enemy replied then both Grey and Erza both attacked next and Lucy saw what looked like copies of the wizard appeared from nearby and attacked Natsu,Grey and Erza.
"I see" said Lucy "when he's barrier takes a lot of pressure he summons clones of himself to reduce the pressure that's smart" when the three recovered Lucy called them over and told them what she found out watching them attack just now "okay it looks like when you three attack at the same time I'll summon as many of my celestial spirits to defend you", "okay" replied the other three.
Natsu,Grey and Erza attacked at the same time again and it looked like a repeat of last time but Lucy quickly summoned her celestial spirits one after the other blocking each clone from hitting the other three and some of the clones turned to attack Lucy with magic but her celestial spirits protected Lucy and retreated back the Celestial Spirit World to heal.
Because of Lucy's plan the enemy's barrier got destroyed and Natsu hit him really hard and the skin of the enemy ripped off to reveal that it was a robot disguised as a wizard "ah that explains why it killed Aquarius and not my other celestial spirit's it's because it didn't want to get wet" Natsu was about to finish it off but, "Natsu let me finish it off because I want a bit of payback for yesterday" said Lucy, Natsu was surprised but with a smile on his face nodded and said "his all yours he was your target after all", "thanks" replied Lucy.
Lucy took out a key and summoned Wendy Aquarius, when she appeared Wendy Aquarius was surprised and said "that felt weird I'll have to get used to being summoned by your key", "wait why does Aquarius have different colour hair and why does she sound more friendly?" asked Erza confused "I'll explain everything after Aquarius defeats this robot" replied Lucy and she asked Wendy Aquarius to attack the robot blasting it with water shutting it down for good.
"Okay you asked why Aquarius has different coloured hair and why she sounds more friendly? well this is actually Wendy taking Aquarius's place after she died" replied Lucy "WHAT DO YOU MEAN THAT'S WENDY?" all three asked and Lucy explanied everything that happened the day before and being teleported to Celestial Spirit World and what happened there.
"Yeah I'm one of Lucy celestial sprirt's now" replied Wendy Aquarius with a smile "wow seeing Aquarius with a smile and being polite isn't something you see everyday" said Erza, after that they returned to the Fairy Tail guild and told everyone the great news on their success on the mission and Lucy explained to everyone on what happened to Wendy.
Carla was looking worried "if your now a celestial spirit will it mean that I can't see you much anymore?", "wait a moment I be gone for just a second" replied Wendy Aquarius then a few minutes later she reappeared with a smile on her face and said "The Celestial Spirit King said you can come with me to the Celestial Spirit World as long as you wear this" and Wendy Aquarius used her new magic to give Carla celestial spirit world clothes.
"With these on you can still stay with me in the Celestial Spirit World isn't that great" said Wendy Aquarius "We are best friends of course I'll go with you" replied Carla and before they went back " Lucy you can call on me anytime and I'll appear and help you because it's my new role after all" said Wendy Aquarius with a big smile "we are friends till the end and thank you for everything so far" replied Lucy.
Wendy and Carla waved bye to everyone and they went back to the Celestial Spirit World and from then on Lucy and Wendy Aquarius made a great team in their future battles with Wendy Aquarius learning to combine her new water magic with her healing magic.
The end, I hope you enjoyed this story when I saw the picture that I had edited I was thinking what if Aquarius died and someone took her place and then I thought Wendy could and thats how I came up with the story, It was a little hard to come up with something but I did it and I enjoyed writing it.
https://fairytail.fandom.com/wiki/Wendy_Marvell
https://fairytail.fandom.com/wiki/Aquarius
https://fairytail.fandom.com/wiki/Lucy_Heartfilia
https://fairytail.fandom.com/wiki/Celestial_Spirit_King
https://fairytail.fandom.com/wiki/Natsu_Dragneel
https://fairytail.fandom.com/wiki/Gray_Fullbuster
https://fairytail.fandom.com/wiki/Erza_Scarlet
https://fairytail.fandom.com/wiki/Carla
