Is telling something unpleasant, something that would portray a person in a very negative light even though it is true, considered Lashon Hora?
Loshon Horeh is a very wide scope of offenses performed by saying something. It is not an offense on its own (besides gossip see Rambam's Deot) but an umbrella of all. That includes various prohibitions of deceiving, assaulting, gossiping or simple overriding of "loving thy neighbor".
As your question is not specific and there are many different scenarios for telling the truth that answers your description, for example,
- telling someone that his relative died or
- telling someone you saw him smoking weed or
- telling someone his fly is open or
- telling someone his wife is cheating on him or
- telling a white person he's white and a black person he's black or
- praising someone for his earnings in front of an IRS agent.
Without going through all the details of L"H, I'd say it simple - if you're sure the person will thank you for saying that - say it, otherwise consult a Rabbi.