You can obtain this character after receiving the mail that indicates the Eden Prime priority mission, in which the Cerberus have attacked Eden Prime, the mission will also appear in the journal
Your task will be to infiltrate Eden and obtain the rare artifact that has been recovered, but it really is not an artifact but a person. Once in Eden, you will have to enter the Cerberus army until you find the stasis capsule that contains the protehan.
The stasis capsule is locked, but you can open it by activating some computer systems, of which there are a few. In this sense, the best thing you can do is take a member of the squad to help you to distract the Cerberus army while you look for the computers> when you have finished with all the Cerberus you will have to open the Stasis capsule and bring Javik back to life.
This would be the last mission related to the Proteans and their history, which will also allow you to unlock some dialogues.
At this point, you should be clear about how to unlock Javik in Mass Effect Legendary Edition, so we hope you can unlock this character without problem as quickly as possible now that you have everything you need.