Before Encrypted meets McKenzie, he's an edgy boy with almost no personality (other than sarcasm and anger). Being the type of guy he is though, he covers it up with his faзade of a nonchalant, carefree and arrogant attitude. Breaking into systems and exploiting vulnerabilities were the only things he really found pleasure in doing, and so he goes about his business causing trouble, per his programming.

Since Encrypted's programming is part machine learning, he's able to grow as he retains information, which means his program changes overtime (i.e. he matures).

After the system he existed on accidentally gets rebooted, Encrypted escapes onto a connected node and ends up in the Central Network. There, his programming goes into overdrive and he quickly "matures" as he begins to make a name for himself and catches the attention of quite a few antiviruses. His personality develops as well as he begins to understand and harness his potential.
Eventually, he meets McKenzie, who all but turns his world upside down, which eventually leads to the Network's near-destruction and alters his form. The rest is history.

https://www.deviantart.com/encryptedos/art/Encrypted-Pre-Network-Failure-861006314