E.S. Simeon is Albedo Piazzolla’s E.S. unit. Like the E.S. Issachar and E.S. Zebulun, Simeon uses the weapon system known as Aird. Along with Albedo, it dematerialized half-way through Episode II and while Albedo later reappeared, E.S. Simeon did not. E.S. Simeon's system is known as the Relic Drive. This drive fuels the E.S. by absorbing the 'Will' of living beings. In the Xenosaga: the Animation it directly absorbs a Kirschwasser seen bound to its Vessel of Anima and transforms into a giant bio-organic monster with hundreds of tentacles and a gaping maw. It is reduced to a pool of molten flesh and comments from chaos and Jr. suggest that it didn't have a proper Vessel to begin with.
E.S. Simeon is later seen in the anime's epilogue restored to its original mechanical form. The mech seen piloted by Albedo as the White Testament at the end of Episode II and in Episode III is indeed E.S. Simeon, or more precisely Simeon's Vessel of Anima aligned to a new mech chassis. The new frame has no legs and a series of dragon-like extensions rising from the shoulders. These extensions may be meant to resemble the many heads of the Hydra, which would be fitting considering the inborn ability of the pilot. The new E.S. Simeon is not fought at any point in Episode III and only appears once at Rennes-le-Chatâeu. The shoulders are on an axis and look like they could only rotate forward, and back by a relatively small, amount because of the extensions. This may be to compensate for the fact that the arms don't bend at the elbows. Strangely it has an odd number of extensions 6 on the right, 5 on the left which can be seen if you look closely and count the pink, triangular parts.