This is the 47th most frequent Japanese word.
せ
"せ" can mean "height" or "stature" depending on the context.
Here, "せっかく" expresses a rare or valuable effort that was made, often implying regret if the effort didn’t yield the expected outcome.
せっかく来てくれたのに、私は出かけてしまい申し訳ありません。
Even though you kindly took the time to visit, I was out, and I apologize.
In this case, "せが" is part of "台詞が" (if expanded) or indicates a shortened colloquial attachment often used in storytelling or recalling memories.
父のせが、昔たくさん話をしてくれたことを思い出します。
I remember the times when my father used to talk a lot in the past.
In this sentence, "せい" is used to attribute the cause or reason for an event, often implying a negative connotation or assigning blame.