Support relationships are a game mechanic that first appeared in Fire Emblem: Fuuin no Tsurugi on the Game Boy Advance. (However, the love system in Fire Emblem: Seisen no Keifu established many of the concepts that would solidify in the support system.) A support relationship is a special bond between two player characters that grants then stat boosts when in close proximity to each other on the battlefield. In most games in the series, these support relationships may only exist between predetermined pairs of characters, but in Fire Emblem: Radiant Dawn, the system is opened up to allow any two player characters to form a relationship.
To form a support relationship, the player must use the characters in the desired relationship in battle together. Depending on the game, it may be sufficient that the characters merely participate in a specific number of battles together, but some games instead require that the pairs end a specific number of turns while standing adjacent to one another. Once the requisite number of turns or battles have passed, the two characters may then engage in a support conversation. Upon having the conversation, the support rank between the two characters will increase by one level. Characters are limited in the number of support relationships that they may engage in, and it is not possible for characters to max out more than one relationship at a time.
In addition to gameplay benefits, support conversations also allow players to see sides of the characters not seen during the course of the main narrative. As support ranks between pairs rise, their emotional bond also grows closer, leading to a deepening friendship, greater admiration for one another, and in some cases, romance. A character's specific ending may also vary depending on whether they have reached the maximum support rank with a specific character. For example, in Fire Emblem, Eliwood can engage in support relationships with several female characters, including Lyn or Ninian. Should he max out a support relationship with one of them, the ending will state that the characters eventually marry.