Riya's spells cause him to transform his body parts to use for battle. Which proves useful for his partner Alishie, as Alishie is knowledgable enough to use it to its advantages. Their strength and limitless determination make them a very powerful and productive team. They prove to be a useful group in the of taking down Faudo.

Though it was not revealed how he found Alishie and why he did so, it is shown in one of the covers of the manga that Riya's first meeting with Alishie was when Alishie stopped a giant boulder from crushing a few children from his village. It can be speculated that after seeing Alishie save the few kids from a giant boulder, Riya was inspired to protect others as well and chose Alishie as his partner.

(Source: Zatch Bell! Wiki, edited)