"Buttons" for Lissa, as a result of her unusual hat.
Masked Marth has been dubbed "Charth" in some circles.
The game's downright bizarre renditions to Armor Knights, like Kellam, have been dubbed "Shellfish" or "Shellfish Knights".
NoNo for Nowi, based on her Japanese name Nono. Guess why.
"n_n" for Nah, due to the impossible-to-spell nature of her Japanese name.
After the revealing of his English name, the great conqueror Walhart took all of about two seconds to be forever known as Walmart.
Gerome's Domino Mask, black color scheme, and grim disposition have lead many to call him "Batman". Or "Char" if his father passes down blonde hair. Quite a lot of Nightwing as well.
Thanks to the fact that Frederick is the only character who can survive the earlier parts of Lunatic and Lunatic+ mode, the early stages of the mode are called Frederick Emblem.
"Prince Morgan" and "Prince Inigo" if Chrom is their father. Less so "Prince Brady", "Princess Cynthia", and "Princess Kjelle", as those names see much less frequent use.
Prince Marth, due to character limitations, was localized as "Pr. Marth". Generally he'll be referred to as anything else that contains those two letters; the most common interpretations are Professor Marth and President Marth, but imagination has been people's only limit - there have also been Public Relations Marth, Prudish Marth, Prosecutor Marth, People's Republic of Marth, et cetera.