Frankie is referred to by George as "the Summer me"; he refers to George as "the Winter me". Jerry went to camp with Frankie instead of George every year.
Frankie has been known to be very sensitive. His nickname is "Fragile Frankie Merman", as whenever someone did something that hurt him, he would run into the woods and dig a hole for himself and cry for hours. He is shown to still harbor this personality.
The Junk Mail
In “The Junk Mail”, Frankie promises Jerry a new car, but gives him a large van (as kids, Jerry and Frankie dreamt of driving around in a van). When Kramer lies about having the van, Frankie is offended and runs into the woods. George parks the car in the woods, waiting for his parents, but Frankie sees George has the van, and runs shouting "Seinfeld's Van!", which George mishears as "Son of Sam". After running off to dig a hole and reuniting with Jerry, George, and Rhisa, he is with the group when they open the van doors to find Frank and Estelle expressing their “love for one another.” After this, he tells Jerry to sell the van. Jerry agrees, “Oh, yeah.”