鏡音レンの暴走 (Kagamine Len no Bousou)

Background
This song is about Len talking to the listener about what he feels for his Master and how he is perverted towards him, and expressing his anger for him (at one point telling him that if he hates onions, to go stick an onion up his ***). He also talks to Rin and Miku about his feelings as well. He says how even though he is short with bad vocals, he will show everybody what he is made of. He eventually goes on a road roller rampage.

This song also popularized the concept of Len being a "shota" character.

This song was uploaded on the anniversary of cosMo's first upload, and stayed in first place in VOCALOID Ranking for two weeks in a row.

This song is featured in the EXIT TUNES compilation album EXIT TUNES PRESENTS Vocaloanthems feat. 初音ミク

Trivia

 * Part of the lyrics are actually borrowed or recycled from Hatsune Miku no Bousou, one of the first work of cosMo.
 * The attire that Len features in this song is a combination of a school swimsuit (sukumizu) and a typical majokko outfit. Both are often fetished outfits in the anime/manga genre. The fact that Len is dressed in this clothing reflects how perverted Len's Master indeed is. The attire itself became quite popular in the VOCALOID culture and frequently is used in puns or jokes to humiliate Len.