The reading カ derives from the phono-semantic analysis (the same reading as 哥). The origin of the reading セイ is much unclear, the first known attestation comes

The reading カ derives from the phono-semantic analysis (the same reading as 哥). The origin of the reading セイ is much unclear, the first known attestation comes from IBM's conversion table [1]. [1] http://www.asahi.com/special/kotoba/archive2015/moji/2011081800015.html http://www.asahi.com/special/kotoba/archive2015/moji/2011081...