必 - I Will, WITHOUT FAIL. Cross My Heart.

Readings & Meanings:

かなら（ず）

ヒツ

without fail

Common Usage:

必ず

必要

必死になった

必勝

Examples:

借りたお金は必ず返した方がいいに決まっている。

必要に迫られて、資格を取った。

彼は必死になって皆に付いて行った。

"必勝"の鉢巻を巻いて受験勉強に取り組んだ。

Translations:

It is always best to return the money you lend without fail.

Necessity urged me to get the license.

He was desperate to catch up with the others.

With a rolled up "certain victory" headband, he wrestled with the exam studying.

Radical:

Despite being crossed out, 心 is our radical.

Also, despite being a Kokoro-radical kanji, the pictograph had nothing to do with a heart. It used to depict a slab of wood, pressed between boards to be straightened. It couldn't go anywhere like that, so the meaning was "sure, certain."

But I like to remember 必 by thinking "Cross your heart and hope to die," the old children's rhyme that means "I swear."

References:

http://www.kanjinetworks.com/indices/radindex.html
http://www.thejapanesepage.com/node/kanji/7.htm