Rave Master

Rave Master (called Groove Adventure Rave or simply Rave in Japan) is a previous manga of the author of Fairy Tail, Hiro Mashima which ran in Weekly Shonen Magazine from 1998 to 2005. Which tells the story of Haru Glory, a teenager who becomes the second Rave Master and is destined to bring peace to his world and save it from the threat of Dark Bring (Shadow Stones in the English version). Along the way he gains friends who join him on his quest and slowly unravels the mystery of the Rave Stones (his power source), The Dark Bring, his destiny, how it affects those around him and the fate of the world in general.

Many elements of this series pop up in Fairy Tail often. Many characters have similiar designs as Rave Master characters. Most notably:

Character Resemblance

 * Plue, Haru's guide, being one of Lucy's Celestial Spirits
 * Lucy Heartfilia resembles Elie
 * Natsu Dragneel resembles Haru but Let in fighting style. Besides, When Natsu becomes the Great Demon Lord Dragneel in Edolas, he resembles Lucia Raregroove, the final antagonist in Rave Master.
 * Gray Fullbuster resembles Musica, and he wears a necklace that bears resemblance to Haru's Ten Commandments sword or the first piece of RAVE.
 * Erza Scarlet´s Re-equip Magic resembles Leopard´s Dark Bring (member of the Six Guards of the Blue Guardians), which allows her use anything (even elements like wind, electricity or fire) and equip her like a armor and use his attributes against her opponents (Fire: burn, Electricity: stun, Wind: speed increased).
 * Jellal Fernandes is based off Sieg Hart
 * Faust resembles Shakuma
 * Mirajane (when she was younger) resembles Julia
 * Aetherion is taken from Etherion and is a highly destructive magic in both series
 * Karen Lilica resembles Reina of the Oracion Seis.
 * Gildarts Clive takes design elements from Gale Glory and Gale Raregroove
 * Levy McGarden resembles Belnika, except with a smaller chest
 * Hades resembles Haja.
 * Fried Justine resembles Alpine Spaniel, member and leader from the Knights of the Blue Sky in hairstyle and face, except that Fried don´t have a beard like Alpine.
 * Azuma´s appearance resembles that of Pumpkin Doryu, one of the major enemies in Rave Master. His Earrings also resemble Doryu's Dark Bring Vampire, and his magic is very similar to that of the Six Star Dark Bring "Ballenttänzer Zeffrea", which was possessed by Shuda, a character from the manga Rave Master.
 * Aquarius resembles Celia, a character from Rave Master. Both are mermaids and specializes in Water Magic, and shares the love by a ´´boyfriend´´: Aquarius with Scorpio and Celia (a one-sided love) with Haru. The big difference are hers personalities, Aquarius is a very moody and hostile the most time (except with Scorpio) and Celia is very friendly (althrough she have a rivalry with Elie by the Haru´s love, even so, they are still friends).
 * Lisanna´s supposed death in Fairy Tail is similar to the Resha Valentine´s supposed death in Rave Master. In both cases neither was really death, both was alive.
 * Elfman has a striking resemblance to Franken Billy. Also, Both have a power that attributes his arms (Elfman with Take Over magic, and Billy with a Dark Bring).
 * Laxus Dreyar' design and personality seems to be based off of Hardner, a similar, sadistic character and leader of the Blue Guardians.
 * Cobra´s appaerance resembles Shuda (a former Oracion Seis member and later Rave Warriors´ally). Coincidentally, Both characters (Cobra and Shuda) were Oracion Seis members in his respectives series.

Rave Appearances
One can also spot characters from Rave Master in the background. The cover for chapter 91 even featured Lucy and Natsu dressed as Haru and Elie from the series.
 * Erza's armor has the same 'Heart Kreuz' logo as Elie's tank top.
 * Erza Knightwalker's Saint Spear the ten comandments transforms into Silfarion, Mel Force, Explosion, Gravity Core, Blue Crimson, and Rune Save, all with identical abilities.
 * Casino in Chapter 75 has a Ruby's figurehead.
 * The brand 'Heart Kreuz' is featured prominently (All Requip Amour has logo featured in some way or other, Lucy's Clothes, and both Erza and Lucy have Heart Kreuz Bags, Suitcases, etc.)
 * In the Omake Lucy's Photo shoot, Elie, Julia, Belnika, Musica, Ruby and possibly Griff (because it is of the same species as Griff and it has a perverted look on it's face) make a small appearance.
 * Griff makes appearances often, the most prominent being in the anime where Happy is chewing him instead of candy.
 * Griff explain Mest's memory magic in chapter 215.

For more information of the series, check the Wikipedia article on Rave Master or its Wikia site.