The letter has been given to person concerned.

信件已交与本人。

《新英汉大辞典》

The person concerned is totally unaware of what is going on.

对发生的与自己有关的事情一点儿也不知道。

youdao

Eg: The person concerned requested no revealing of his name.

例： 这位当事人要求不披露他的姓名。

youdao

And think about this in that special first person ego-concerned way that comes naturally to us.

以一种自然的自省方式,来思考这些试验。

耶鲁公开课 - 死亡课程节选

Later that day, the first victim issued a statement to the effect that the person he described as "the individual concerned" was not the person he had previously identified as Lord McAlpine and apologised to Lord McAlpine and his family.

BBC: Ken MacQuarrie report: Summary of findings

The DUP's Peter Weir was concerned about "the person who finds themselves half a mile down the road from a new prison".

BBC: Justice Committee

"We are always concerned when someone illegally accesses another person('s) or government agency's network, " he said.

CNN: Anonymous threatens Justice Department over hacktivist death