Answer:
周 SHUU, mawa(ri) - round. loop. lap
喜 KI, yoroko(bu) - be glad
昆 KON - elder brother, later, insect
思 SHI, omo(u) - think, believe
皮 HI, kawa - skin, pelt, leather
They all take the "fish" radical (魚 GYO, sakana, uo - fish) to become other chracters!
鯛 tai - sea bream
鱚 kisu - whiting
鯤 harago - a mythical giant fish
鰓 era - gill
鮍 kawahagi - thread-sail filefish