That person has been working for this office for many years and is virtually in charge of this company.
That person has been working for this office for many years and seems to know everything about it.
That person has been working for this office for many years and is virtually in charge of this company.
の方は、「あの人はこの事務所に長年勤めていてこの会社の主みたいな存在だ」となりまして、
That person has been working for this office for many years and seems to know everything about it.
の方は、「あの人はこの事務所に長年勤めていてこの会社について全てを知っているようだ」となります。
That guy has been working here for decades, it's like he lives here.
ここの「主」は a long-term employee のように言えますが、次のように言った方が、「主」感が出て良いかなと思います。
ーThat guy has been working here for decades, it's like he lives here.
「あの人はここで何十年も働いているので、ここに住んでいるかのような」＝「ここの主のようだ」
ご参考まで！
日本 
カナダ 
