Please enjoy this thought;
So, Kat and Seto have been in the same Dueling Circuits since they were 13. He has known her her entire Dueling career, and, with him kind of taking a bond towards Max initially as the creator of the game and having some form of at least business relationship bw I2 and KC, he has gotten to know a lot of her mannerisms.
Besides, they were each others rival until Yugi was in the picture. He studies his opponents well.
So, picture, if you would, a situation at some point in the fic where Kats panicking over something and Seto finally just goes; "Kat, what is the thing you always say when everything is going insane?"
"I...I dunno--"
"Yes, you do, do not make me say it, it's ridiculous."
"...."
"...Kat."
"...Maybe you should say it, it would make me feel better."
"Not a chance in hell."
"SETO COME ON, I NEED SUPPORT-"
"NO."
"PLEASE."
And there is just a deep pained sigh. "...W.W.D.D."
And the gang, whether they know it or not, are going to egg it on until he finally says
"...What would Dolly do?"
