Idols who are fluent in English. Some idols who speak fluent English include Bangchan and Felix (SKZ), Han and Seungmin (SKZ) are near fluent; Rose, Jennie, Lisa (Blackpink); Yuqi and Minnie ((G)I-DLE); Johnny, Mark, Ten, Jaehyun, YangYang, and Hendery (NCT); Wendy (Red Velvet); Lia (Itzy); Yoohyeon (Dreamcatcher); BM (Kard); AleXa, Jessi, Eric Nam, Amber Liu, CL, Chungha, and Jeon Somi (all soloists)
Edit, more idols who are fluent in English: Jae (Day6); Byeongkwan (ACE); Sorn, Eunbin, Yeeun, and Elkie (CLC); Wonyoung (ex Iz*one); Giselle and Ningning (Aespa); RM (BTS); Ailee; Joshua and Vernon (Seventeen); Minzy; Sunmi; Ahin and Nancy (Momoland); Daisy (ex-Momoland); Key (SHINee); Mark Tuan, BamBam and Jackson (Got7); Hani (Exid); Tiffany Young, Yoona, Seohyun (SNSD); Jessica (ex-SNSD); Eric and Kevin (The Boyz); Keeho (P1Harmony); Yeonjun and HueningKai (TXT); Lay (Exo); Haseul (Loona); and soloist Jamie, plus a lot more that I missed.
In short, a lot of idols can speak English really well, because they're fluent! However, nonfluent idols that still speak English well are Hyunjin (SKZ); Jisung (NCT, he understands a lot, but doesn't speak it much); Jisoo (Blackpink); Twice's Mina has really good pronunciation, as well as Nayeon, Jihyo, and Chaeyoung.