Pretty much a perfect use of that portrait.
It is pretty much perfectly fitting - the problem is coming up with portraits for the enemies, though... There's a serious lack of portraits in the assets in general.
And for the record, there is a serious lack of characters viewed from the back sprites as well. The isometric mech was pretty much the only thing besides some low-res RPG sprites which didn't really look all that good plus they wouldn't have worked together with the main character portrait. It'd be a lot nicer to have the main character flying than having him be inside a flying machine. Using any of the shmup ship sprites is out of the question since they totally don't work together with the 3D background progression (which is pretty fancy-looking even if I say so myself).
I've thought about using 0rel's cut&paste assets for lot of the portraits but their lack of color is quite bleh. Oh well.
I intend to get some actual content done by Monday and then put up some gameplay video.