I thought Gary Stu was the male version of the Mary Sue term.
Also maybe it is just me but I do say that Ephraim and Alm are Gary Stus.
Ephraim takes all the army of Grado takes a castle and survives an impossible situation, being completely surrounded and outgunned, out of his butt and with two cavaliers.
And Alm, for all SoV calls that you should always try to take the middle path and how commoners are as worthy as nobles the story always seems to put Alm on the right or at least as better oriented than Celica.
"We don't need gods" "We need to fight for Zofia" or how Alm is the prince of half of Valentia, that kind of stuf.
Ps: In my honest opinion Celica gets the shaft in SoV.