How did such an evil presence get into a place that was both a material & spiritual Paradise?
After all, Hashem didn't put it there because only He only placed Adam in the Garden, not the Nachash.
In Chapter 12 of Rav Yehudah Petiyah's Minchat Yehudah in the section on Parshat Beresheit, he asks rhetorically:
Did it happen when all the animals passed before Adam to receive their name?
No, Rav Petiyah answers his own question. The Nachash clearly heard the prohibition against eating from the Eitz HaDaat (Tree of Knowledge), which occurred before the naming of the animals and even before the creation of Chava (Eve).
Clearly, the Nachash was in Gan Eden prior to all that.
So...when & how did that happen?
Be Careful with whom You Align Yourself–You May Not Share the Same Motivations
Adam called the Nachash into service to help him care for the Garden. After all, the Nachash possessed more skill & talent than any other animal: "He knew how to speak like a human being, and was more cunning than the fox, and possessed more strength & power than the lion..."
And when Chava became her own person, the Nachash went to serve her too.
And that gave it the opportunity to tempt her into eating from the Eitz HaDaat.
It's sort of like the famous 1986 quote from former President Ronald Reagan regarding "the nine most terrifying words in the English language":
"I'm from the government and I'm here to help."