既是

既 = 旡 + 艮 : Mnemonic symbol: the saying 一言既出，驷马难追 means "lit. once said, a team of horses cannot unsay it (idiom); a promise must be kept". Thus, I'll represent this character with a team of four horses.

Joan of Arc made a bet with a team of four horses that she'd be able to chew 20 chewing gums at once. After stuffing ten of them into her mouth she's already chocking, but what has been said cannot be retracted by a team of four horses.

是 = 旦 + 龰 : Mnemonic symbol: a skull, from "to be or not to be".

Sherlock Holmes (sh) starts his day in the space station's bathroom (Ø4) with a mug of coffee (旦). Holding a skull (mnemonic symbol for 是) in his hands, he asks himself "to be, or not to be" (是). He wonders if he can make the skull come back to life using lots of coffee, and injects coffee from his mug into the skull using a syringe (龰).

是 = 日 + 下 + 人 : Mnemonic symbol: a skull, from "to be or not to be".

Sherlock Holmes (sh) watches the Neanderthal Man (人) trying to climb to the sun (日) on a ladder (下) in the space station's bathroom (Ø4). Sherlock is afraid that he'll open the window, so he puts on a big skull (是) and scares him off.