Len and White Len Altered Characters
Len and White Len altered characters (レン(白レン)改変キャラ, Ren (Shiro Ren) kaihen kyara) refers to modified versions of Len and White Len (sometimes made into a new character entirely) made for MUGEN.
Len originally appeared in TYPE-MOON's visual novel Kagetsu Tohya, a sequel game to Tsukihime. White Len appeared in the fighting game spin off Melty Blood (with Len also being in it). But due to a combination of factors, they became very popular in MUGEN. As a result, a wide variety of altered characters have been created, just like the various Shiki altered characters.
There are many altered characters that are not depicted in pixiv even today.
Xehrnyth
She's fast with some attacks doing a lot of damage, but her defense is really low. Meaning, a normal combo can deplete her life really fast.
Alt
A Len who is a heavy weapons carrier. Her mobility is sluggish with each blow being heavy.
Kouya Len
Baseball player Len who uses baseball themed attacks and can hit balls thrown at her.
Pale
She's slow but has attacks that makes her teleport near the opponent and long range attacks that makes up for her speed.
She is the one with the blueish-green tone in this image, the Len next to her is Dim
N-Len
Styled after both Nero Chaos and Neco-Arc Chaos. She summon some characters from different games and animes.
Ren
Styled after The King of Fighters' Oswald who controls flames and explosions. She has a slightly mischievous personality and is a smoker.
She started out with the same hairstyle as Len, but later changed it to a long, loose braid.
MUGEN M.U.G.E.N. Melty Blood Kagetsu Tohya Tsukihime Len White Len Shiki Tohno and Nanaya Shiki Altered Characters TYPE-MOON