I have not used mnemonics for any of the Japanese alphabet for years as those are introductory character sets to learn within a time-frame of four to six weeks (longer in grade school).
Here, the hiragana character pu (ぷ) is an augment of fu/hu (ふ) by adding a maru (まる; circle) to the top-right corner.
If I HAD to use a mnemonic, the base character looks like an upside down, abstract question mark. fu/hu in Japanese is pronounced similar to one of the five W's: "who?", but with slightly more emphasis on the introductory "wh" sound, like lightly blowing air. After all, a question mark is often something that follows when "who" is used at the beginning of a sentence in English.
Any of the characters in the ha/hi/hu/he/ho (は/ひ/ふ/へ/ほ) family can be augmented to pa/pi/pu/pe/po (ぱ/ぴ/ぷ/ぺ/ぽ) by adding maru. It is the only hiragana/katakana character set that can add maru to change pronunciation.
