The Mera'din, or "Brotherless" in the Old Tongue, are those of the Aiel who could not accept either Rand as Car'a'carn, or accept his revelations about their history. They fled to join the Shaido (ACoS, Ch. 40). Many do not trust them (ACoS, Ch. 40) and seek to take advantage of them, but the Wise Ones do not allow this (TPoD, Ch. 11). When Sammael disperses the Shaido, Sevanna arranges to have as many of the Mera'din with her own Sept, the Jumai, as possible, as she did not trust him (TPoD, Ch. 11). It is mostly Mera'din and Maidens who capture Faile and her party, and the Mera'din give aid to them, which the other gai'shain think should be shaming (WH, Ch. 2). The Shaido in general have a low opinion of the Mera'din (WH, Ch. 2) and the Mera'din come to have a similarly low opinion of the Shaido, such that before Perrin captures Malden they are considering returning to the Waste on their own (KoD, Ch. 5).
"The rush of his sept became a flood of algai'd'siswai he did not really trust, men who had fled their own clans because they did not believe Rand al'Thor was truly the Car'a'carn. Maeric was not sure what he himself believed, but a man did not abandon sept and clan. These men called themselves Mera'din, the Brotherless, a fitting name" (Maeric, A Crown of Swords, Chapter 40).
"We cannot follow a false Car'a'carn and a wetlander at that, but perhaps we will be allowed to live out our lives in our own holds. We think on it. We have been away from home a long time and these Shaido sicken us" (Knife of Dreams, Chapter 5).
"The Brotherless had been men of honor; though they'd called themselves cast off, they had been the only ones in Malden who had maintained the old ways. When the gai'shain women had started to be in danger, the Brotherless had chosen and protected those they could. They hadn't asked anything for their efforts. Well...that wasn't true. They had asked for much, but had demanded nothing." (Faile thinks about the Brotherless; The Gathering Storm, Chapter 21)