Alba: Silence! Don't you ever address me with such a flithy word again.
Juniper's Knot: When the tired boy falls asleep in front of the demon girl, she is suddenly struck by her bitterness and frustration on how she cannot sleep, escape, or in any way find respite. The emotional baggage makes her lose her composure and throw every profanity she can think of at the sleeping boy, even though he can't hear her.
Grisaia no Kajitsu: Has one instance in Irusu Makina's route where Sachi - a prim-and-proper, if OCD maid who's sole objective is to be a "good girl" - is pushed a bit far in a mock exercise regimen by the protagonist Yuuji and suddenly starts violently cursing at him. With anti-Japanese racial slurs. In English. Also counts as Cluster F-Bomb.
Nine Hours, Nine Persons, Nine Doors has a a good number of uses of strong language, including "fuck", although most of them are said by the same character, Santa. However, when they're in the incinerator, Lotus, who's completely in the dark about the backstory to what's going on, drops one of her only strong swears.
Lotus: "What the hell?! What! The! Hell?! What are you talking about?! It's "9 years" this, and "9 years" that, and when it's not 9 years something, then you're talking about some sort of fucking "experiments"! What in God's name are you talking about?!
When the surviving characters are running up the stairs to escape in the true ending, Junpei hurriedly explains to Seven and Lotus about hexadecimal, bases, and about what would happen if you go all the way to base-22. This leads to the two finally realizing the big twist revelation
Junpei: That wasn't a  on the door!
Seven: It was a Q! A fucking lower case Q!
In Sickness, this occurs Upon Sara pulling a knife out on Violet, who is trying to seduce Suoh into leaving the organization:
Violet: You fucking bitch! You weren't supposed to be here!
At one point in the Emi path of Katawa Shoujo, Hisao, when feeling particularly bitter about their relationship, thinks of them as "friends who happen to fuck."