A spoken English equivalent of 'someone who has glass-dust'

Question

In my native language we have the expression having glass-dust by which in spoken language we describe a person who's generally good but sometimes gets dishonest to take advantage of a situation or does an immoral thing now and then.

Don't trust him completely. I know him for years. He's got glass-dust.

Or

He's a good bloke but he's got alittle glass-dust.

Is there a common spoken form of this expression in English? (Well I know how to use adjectives to express the idea but any spoken idiom or expression?)

I'm asking this because I was watching Southpark the episode in which Tweek & Craig were thought to be gay but they aren't so they pretend that they're breaking up in front of everyone. Tweek said,

"I opened myself up and let you in, but you've got spikes, man."

It came to me that this is exactly where a person in my native language would say, "you've got glass-dust, man" to imply that you're dishonest and duplicitous.

I searched dictionaries but I didn't find anything like have spikes even as a spoken expression. As native speakers, do you think it's acceptable as the equivalent I'm looking for? If not, what do you suggest?

Edit: We say someone has glass-dust (in nature) as there is a piece of broken glass in food. Food is generally good but if you're not careful you can cut yourselves with that piece. I figured maybe spikes represents glass-dust in my native language. But I couldn't find anything to back that up.

Answer 1

From the context you gave, the statements from Southpark are related to the analogy that we all have a soft inner (emotional) self that we hide behind a hard outer protective (emotional) shell that we show to strangers. The part "you've got spikes" means that while the speaker opened himself up in the relationship, the other person's personality caused damage to him. I don't believe this is the same meaning as your phrase "he's got glass dust" in your language, since it refers to a specific kind of damage (emotional) that was caused in a specific type of relationship, and not a general tendancy to make 'bad' choices sometimes.

Instead, the phrase "He looks for the easy path" implies someone who might do the right thing, but often chooses to do the wrong thing instead (because that choice is easier for him). Another alternative is to say "he is unreliable", which implies that he may be reliable at times, but often is not, so extending trust to him is a bad idea.