This papercraft is the Core Drill, based on the mecha anime TV series Gurren Lagann (Tengen Toppa Gurren Lagann), the paper model is designed by Kreuzer. The Core Drill is a small, key-like drill that Simon finds immediately after the series prologue, and is almost always seen in his possession or in the keyhole powering Lagann. The Core Drill is shown to power Lagann, though Lagann is also shown to be able to act on its own and use spiral energy without it. The Core Drill can also sometimes be seen glowing from within with spiral energy.
The Core Drill is also not the only one of its type, as Lordgenome knows of its existence, and by the fact that the rest of Team Dai-Gurren is able to obtain one for each member to power a Lagann during the final two episodes of the series.
Simon finds the Core Drill in the dirt while digging a tunnel in Jeeha Village. He immediately stops his work and leaves the tunnels, and bumps into Kamina, who reassures him that "... drills are your soul, Simon."
Later, after bringing Kamina and Yoko to the unearthed Lagann, Simon figures out that the Core Drill is some sort of power source for Lagann, and uses it to start the mech. From then on, it is almost always seen powering Lagann or around Simon's neck. The Core drill is then passed on to Gimmy Adai after the death of Nia and Simon's chosen exile.
You can download this Gurren Lagann paper craft model here: Gurren Lagann - Core Drill Free Papercraft Download