noun
greenmailer
A person or company that buys a large stake in a target company and threatens a hostile takeover to force the target to buy back the shares at a premium. This is a specialized financial term.
その投資家はグリーンメーラーとして知られ、企業に高値での自社株買いを迫った。
The investor was known as a greenmailer and pressured companies to buy back their own shares at a high price.
グリーンメール refers to the practice of greenmail itself, while グリーンメーラー is the person or entity that engages in it.
Borrowed from English 'greenmailer', a compound of 'greenmail' (a portmanteau of 'greenback' and 'blackmail') and the agent suffix '-er'.