The Melior Esse are basically the combat generals from the Ruina army, original enemies that were created for Super Robot Taisen D. Melior Esse means "Superior Presence" in latin.

The Melior Esse were created by Perfectio as to arrange the foundations of his forthcoming to this existence plane, basically he needed to be feed with as many negative feelings as possible before being able to materialize himself. It could be surmised that Perfectio only saw them as disposable tools, as neither of them are meant to live particularly long life spawns.

The Ruina were meant to have no emotions, however pretty much all of them developed the ability to show feelings through the course of the story, not all feelings are positive ones, though; for example Ignis first developed a budding almost fraternal relationship with Glacies as they were created at the same time, that feeling also caused Ignis to resent Josh since the encounters that he had with Glacies started to change her, and after Glacies betrays the Ruina Ignis develops a full blown hate for Josh "for making her weird". In that sense Contagio & Umbra were the most loyal Ruina retainers as they never gave sings of developing any feelings that would get in the way of their job and were always wholly dedicated to their original duty. Aquila's feelings were "the will to show his combative spirit" and loyalty to "his king".

BTW, Ventos was not one of the combat general of the Ruina, even though he was meant to be one of the Melior Esse; that's probably a side effect of having being seen as a failure by the rest of the generals. The video is lacking Glacies and Ventos as they don't stick to the Melior Esse till the end.

Due to Perfectio's nature, it might be not an outstretch to think that the Melior Esse originally were the crew in charge of investigating the Fabra Fores, but that wasn't really ever touched upon.

The characters shown in this vid are Contagio piloting his Violaceum , Aquila and his Fortis Ala, Ignis and his Impetus and Umbra with her Priscus Nox As a trivia, I'll tell you that not only Umbra is a girl, but her voice actor is also female (it's Ushida Hiroko).

As usual, we need to stretch a huge thank you to nico user koni who gave himself the effort of creating this attack compilation [ koni 's mylist here : http://www.nicovideo.jp/mylist/34821656 ]

Title : 第2次OG　メリオルエッセ全武装

Source : http://www.nicovideo.jp/watch/sm19536651

Description : ウィオラーケウム　フォルティス・アーラ　インペトゥス　プリスクス・ノクス
第2次OG戦闘アニメhttp://www.nicovideo.jp/mylist/34821656