The Lagann is Simon's personal Gunmen and the first one discovered by the Gurren-dan. It has the appearance of a large head with brain exposed and arms and legs attached.
It is revealed in the eighth of the Gurren Lagann Parallel Works videos that the Lagann is one of several Gunmen found by spiral warriors after their battle with Anti-Spirals. Lord Genome had the Lagann buried deep beneath the earth, until Simon eventually unearthed it. It turns out that the Lagann is a unique Gunmen in its ability to adapt itself to control other gunmen, including the Gurren, the Dai-Gunzan, and others.