BTOB's Changsub is asked about his feelings toward Sungjae.

On January 2, Changsub, who recently released his first solo mini album 'Mark,' guested on SBS Love FM's 'Unnies' Radio.' 

DJ Song Eun Yi asked, "I get envious when Kim Sook does well. Do you also get envious seeing Sungjae with great popularity?" Changsub replied, "I'm not envious at all," and expressed affection for the fellow member, saying, "Sungjae played a big role in bringing up BTOB where we are now."