In the story of Fairy Tail, Lucy Heartfilia is a Celestial wizard, and she can use her ten astrology spirits and other magic spells to fight against any foe. She is a flexible mage, since each of her spirits, from Aquarius to Taurus to Sagittarius, has a unique effect. She is a clever, smart, and loyal wizard, and is often seen traveling with Natsu Dragneel the fire mage, and Erza Scarlet, the swordmaster.
Suppose Lucy had a dangerous mission lined up, and she needed to recruit an ally from other animated series to back her up? She needs someone who is patient and loyal, and this teammate should be able to protect her while she uses spirits (Lucy can't fight well up close). Some anime characters are cut out for the job, but not others.
10 Team Up With: Naofumi Iwatani (The Rising of the Shield Hero)
The shield-bearing hero Naofumi Iwatani is an easy choice of a partner for any summoner-type wizard or another squishy mage. He has very few offensive abilities, but in the right team, he doesn't need to. Naofumi is there to conjure barriers and shields that protect his allies while they do the fighting.
A wizard-like Lucy would love to have a defense-oriented ally like this, and although Naofumi is rather gruff compared to Lucy's usual allies, he would work well with her, and they are both smart and combat-savvy. They'd form a seamless team.
9 Reject: Jotaro Kujo (JoJo's Bizarre Adventure)
Jotaro Kujo is a heroic stand user, and he was the star of the Stardust Crusaders story arc. He's a stoic and stern hero despite his age (17), and he's rather perceptive and resourceful too, just like his grandfather, Joseph Joestar.
Lucy wouldn't really like Jotaro very much on a personal level, though, and Jotaro's Stand, Star Platinum, is better geared for offense than defense. Jotaro and Lucy would get in each other's way if they tried to fight as a team, and that's not good news.
8 Team Up With: Bolin (The Legend Of Korra)
Meet Bolin, a talented Earthbender who also has a talent for acting. He is a rather expressive fellow and is easy to get along with, though he's been known to make a few lame jokes or foot-in-mouth comments. No harm was done, though.
As a bender of earth and lava, Bolin is equally potent on offense and defense, and though he lacks his brother Mako's sheer firepower, he can back up Lucy's ranged attacks, and more importantly, he can bend barriers and domes of the earth to protect Lucy from harm.
7 Reject: Levi Ackerman (Attack On Titan)
Levi Ackerman is a Scout warrior, and he is among the very best at what he does. He is a patient strategist, though he has also been known to act rashly or impulsively, usually when personal stakes are on the line. He's also quite ruthless and unforgiving.
Levi's harsh personality would bother Lucy a great deal, and his fighting style is an awkward fit for Lucy's spirit-based combat methods. Levi can't do much to defend her if he's on the offensive, and he's better off fighting with his own comrades against the Titans.
6 Team Up With: Alphonse Elric (Fullmetal Alchemist: Brotherhood)
This hulking armored suit is the alchemist Alphonse Elric, who's the younger brother of the famed Fullmetal Alchemist, Edward Elric. Like Ed, Alphonse has seen the Truth and can perform alchemy without using transmutation circles.
He is also a skilled martial artist like his brother, and Alphonse's alchemy could easily be used to protect Lucy during a battle, anytime Lucy requests it. Alphonse can also guard her with his tough metal body, and on a final note, his personality is warm, making him easy to get along with.
5 Reject: Black Star (Soul Eater)
Black Star is a weapons meister like Maka Albarn, but he's not nearly as humble about it. Black Star's prideful boasts are legendary, and he even claims that he'll work hard to surpass God someday. He also tends to take his allies for granted at times. Actually, he seems to take a lot of things for granted.
His obnoxious attitude would wear out Lucy fast, and his fighting style isn't terribly defensive, either. Instead, he likes to recklessly attack with his fists, which makes him an odd fit for Lucy.
4 Team Up With: Orihime Inoue (Bleach)
Orihime Inoue is a schoolgirl with unique talents. Her hair clips actually contain the spirits of six fairies, and she can use them to cast magic spells. Orihime is a support mage like Lucy, but of a different nature.
Lucy uses her spirits to attack, while Orihime can create very tough golden barriers with her own powers, and her other major spell can heal any wound by rejecting the phenomenon entirely. With barriers and healing, Lucy can stay safe during any mission. She'd also like Orihime on a personal level.
3 Reject: Thorfinn (Vinland Saga)
Thorfinn is a Viking warrior, but he lacks any magical powers since he is based in a real-world setting, from Norway to Iceland to Denmark. He lived in the 11th century and fought with two knives with astonishing speed and skill.
After a point, this bitter and lonely Viking gave up the warrior's life and became something of a wandering pacifist. But in his earlier years, he'd be incapable of defending Lucy against magical foes, and pacifist Thorfinn wouldn't be able to do much of anything. Thorfinn's quest is very different than Lucy's.
2 Team Up With: King (The Seven Deadly Sins)
King is one of the Seven Deadly Sins, powerful knights who find themselves at odds with the rest of the Holy Knights. King is named after his status of king of the fairies, and he can fly around and conjure a plush pillow, arrays of knives and the healing power of magic trees.
King would get along with Lucy fairly during a mission together, and his defensive magic could definitely keep Lucy safe during an intense fight. And if necessary, he can help Lucy attack an opponent with his own enchanted weapons.
1 Reject: Touka Kirishima (Tokyo Ghoul)
Touka Kirishima is a ghoul. She tries to blend into ordinary human society and make friends, but she is a flesh-eating creature deep down, and she can't deny her true nature. Most often, she skulks around at night, agile and aloof.
She is rather bitter and self-absorbed, and she and Lucy would not like each other very much if they somehow ended up on a team. And Touka's defensive capabilities are rather limited, too. She's more of a mobile, flying attacker that needs support.