Mermaid Heel is one of the Legal Guilds in Fiore.
In the AWA fics, Mermaid Heel is allied with the All Worlds Alliance.
In the XP4 fics, Mermaid Heel is allied with the Heroes Coalitions.
Mermaid Heel is known as an all-female guild, with the entirety of its shown members being females. Millianna confirms this when she responds to Erza's question, about if Shô and Wally are in the guild with her. She says" What are you talking about? Mermaid Heel is a guild limited to women." When Hibiki Lates jokingly said that he would have joined Mermaid Heel, Toby Horhorta angrily commented that the Blue Pegasus Mage "is way off their theme".
Mermaid Heel appears to be a considerably powerful guild, whose team for X791's Grand Magic Games made it 6th out of eight squads completing the preliminary event "Sky Labyrinth", which had 105 other competing guilds disqualified. Such an achievement is made more notable by the fact that one of the teams they managed to outrun, representing Fairy Tail, the strongest guild in Fiore, and counting strong individuals such as S-Class Mage Erza Scarlet and S-Class candidates Natsu Dragneel, Gray Fullbuster and Elfman Strauss, made it only 8th during the preliminaries, barely gaining a place to take part in the real Games. Most of Mermaid Heel's power and reputation seems to stem from its strongest Mage, Kagura Mikazuchi, who has been described as being on the same level as Erza Scarlet, one of Fairy Tail's S-Class Mages, as well as the guild's most powerful member.