KanjiJLPT N2
No.

No., residence

Mnemonic

Your younger brother has a strange hobby: he ranks every bamboo stalk in the grove, slapping a No. tag on each one. No. 1 goes to the tallest, No. 2 to the next, and so on. He won't stop until every single bamboo has its proper ordinal label.
Additional thoughts
Visualize younger brother with a label gun, walking through a bamboo forest stamping numbers on each stalk. The bamboo's natural segments mirror orderly progression, reinforcing the numbering concept.
Quick recall
Younger brother tags every bamboo stalk with a No. ranking.

Details

The keyword for 第 is no. (as in "number" or "ordinal prefix"). This kanji is used to indicate order or rank in a sequence, functioning as a prefix before numerals to create ordinal numbers. It conveys the idea of systematic arrangement or ranking, much like the "#" symbol or the word "number" in English when placed before a figure. Historically, it also carried the meaning of a mansion or residence, but its primary modern usage is as an ordinal marker.
On'yomi
だい、てい