Tsurugi was invited by ウエスにぃto anbrella as ShiroAn as diplomatic manager/ambassador. I think this is the best position for Tsurugi, his communication skills and ability to bond to people are his greatest strength and Wesker knows that too, also he understands that Tsurugi won’t enter if it meant he had to do bad things since he promised to his close friends (pein, kakeru, sabu) he won’t. That’s why Wesker proposed the 白An to Tsurugi, he likes the 白Tsurugi.