Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
This is a generic detection for a certain type of Trojan that looks for passwords and steals them once they are running on an affected system.
Malwarebytes can remove Trojan.PasswordStealer without further user interaction. It is recommended to do a full system scan since this Trojan may have been installed by other malware or bundled with more malware on the system. It is also recommended to change all passwords that could have been stolen from the affected system.