I just encountered Leaf/Leif in the World of Light. He is one of my favorite FE characters (Thracia 776 may very well be my favorite game in the series, and my avatar is actually from a piece of artwork form that game). I really liked his match. From Roy being the puppet fighter he was possessing (Roy in general is a similar lord to Leif, especially considering his Sword of Seals/Binding Blade has similar properties to Leif's Sword of Light such as being able to attack from afar and heal its user). The Leif battle had Roy being able to perform random criticals (a reference to how certain skills in FE4/FE5 are able to do just that) as well as him sporadically healing over the course of the match (a reference to the Sword of Light's abilities). It is yet another example of Ultimate making me smile.
I know for some people they are upset over spirits, but seeing Pico and Leaf/Leif get an event match featured about them in a way personally referencing their source material, is something that made me very happy to see. A lot of the spirit battles are pretty clever in how they reference the fighters. Heck, for Pico's battle I particularly loved how it was Bowser with a Ray Gun (since Pico is a turtle-like alien who operates as an assassin) and the fact that Death Wind was the music playing in the background (Death Wind was said to be Pico's home planet). They obviously did their homework for quite a few of these spirit battles.
I think a lot of people are not giving spirits the due it deserves.