From Xingmi (兴迷) to X'BACK (to Beike-r/贝壳r), his solo fandom has been with Lay, back then still only known as Zhang Yixing, since 2005.
Lay's fans were initially called Xingmi (兴迷), which literally translates to "to be crazy/obsessed about Xing".
The character 迷 (mi) is nowadays also loosely used as "fan" when attached to another character, which in this case is the 兴 (xing) from 艺兴 (Yixing)'s name.
In 2016, Lay chose another name for his fandom, which was XBACK. It is an abbreviation for:
Xing
Baby
Alliance
Crown
King
The translation for the full sentence of the abbreviation in Chinese is "兴的宝贝同盟到一起加冕为王", which means "Xing's babies' alliance came together to crown the king".
Chinese fans have been calling themselves 贝壳 (Beike) because the "BACK" of XBACK sounds similar to "贝壳 (Beike)" in Chinese. However, Lay had initially thought XBACKs called themselves that because his fans are very tough, as the literal meaning of 贝壳 (Beike) in Chinese is "shell".
Since he found out that the fans have been calling themselves 贝壳 (Beike), he has also adopted this name for his fandom, adding various little accents to the name for a more personalised flair. Most recently in 2021, he has added a "rolling r" to the end of 贝壳 (Beike), so that it sounds more like 贝壳 r (Beike-r).
Lay first gave XBACKs this name on 24th Nov 2016, which thus marked this date as the fandom's birthday
BEE WITH YOU
陪着你
For X'Back's 5th birthday in 2021, Lay released the single "陪着你/Bee With You", which he specially dedicated to his fans, thanking them for their support and for everything they have been thorough with him, and hoping that X'BACKs and him can continue to accompany each other on this journey.
In 2012, the "L" from "Lay" was made into a hand gesture that means "Love Lay", and it is a way for Lay and his fans to recognise and see each other in a crowd, even if fans were unable to have any fan support lights or items with them. This hand gesture is still used by both Lay and XBACKs to this day.
Over the years, even before he gave us the name X'BACK, Lay has been writing songs for the fans who have been supporting him throughout his journey. It was his way to tell them about his feelings and his gratitude for their unconditional love.