二五仔 rat / snitch

"二五仔" 有兩個意思:

1.   叛徒, 出賣兄弟的人, 又稱"反骨仔"

(俗語) rat ¹
(formal) traitor, betrayer

2.   告密者, 又稱有"鬼頭仔"

(俗語) rat ², snitch
(formal) informer, informant, deep throat
(褒意) whistle-blower

(參考名詞)
針 CI - confidential informant, asset, inside-man
無間道 undercover, spook

References:

rat ¹
"Disloyal person, esp. one who deserts a cause in times of difficulty" Oxford
"one who betrays or deserts friends or associates". Merriam Webster

rat ²
"one who reveals confidential information in return for money". Vocabulary.com
"an informer".The Free Dictionary