必 surely

必 is basically "heart" plus a line crossing it.
He's crossing his heart and swearing to surely never do HITS of acid again.

super important ★★★★★
必 (surely) + 要 (important) = 必要 (super important)

desperately, as if one's life depended on it. ★★☆☆☆
必 (surely) + 死 (death) = 必死 (desperately, as if one's life depended on it.)