Is luck an acceptable character trait?
Question: I've tried to balance it out by having my character's luck failing occasionally and just opting for the least likely outcome rather than something that is beneficial but I'm still questioning whether or not everything will seem too convenient. (Also something I think I should mention, This character isn't the main character.)
You've hit upon the problem with giving a character too much of an advantage. If luck always works in favour of your character, you have to make sure your character can still lose or have bad things happen to him. If his luck automatically makes him win in every situation, it can take all the tension out of a story. Why should the reader worry about whether the character will succeed or fail? Life becomes a slot machine that hits a jackpot every time. You story could become boring and predictable.
Incidentally, the same thing can happen when you give a character magic powers: if you don't built limits into the magic, you end up with no story because everything is too easy for the character.
That doesn't mean you shouldn't use the character. It's an interesting trait. You just need to find a creative way to balance the advantages with disadvantages.
A classic example is the character Cassandra from The Iliad
. She has the power to know the future, which ought to give her people a huge advantage. However, the limitation is that no one ever believes her prophecies.
Another example is the TV series, Stan Lee's Lucky Man
, in which the main character
has a bracelet that makes him powerfully lucky. The limitation is that for every lucky event that happens to the character, an unlucky event occurs in his life to balance it.
In Larry Niven's SF novel Ringworld
, some characters are lucky by nature (they are the product of several generations of human beings who have been bred for luck). However, they don't realize they were bred for this trait. Lots of things happen to them that don't seem lucky at first. They get into a lot of dangerous situations and suffer many of what look like setbacks. It's only by the end of the story that you realize that all these mishaps actually led to an outcome that was extremely unlikely and fortunate -- proving they were lucky all along. It is a risky choice on Niven's part. Some readers might find it disappointing to discover that characters' luck made the entire plot predictable in hindsight.
So I think you're on the right track by putting limits on the usefulness of the character's luck. Just make sure that once you establish the rules/limits you stick to them consistently. Also, it helps if the limits to the character's luck are as significant as the benefits, and if the magic can be a disadvantage as often as it is an advantage.
Often the fun of stories lies in establishing a set of rules that make it difficult for the main character to win. Then, at the crisis, you want the character to find an unexpected way to win that doesn't violate the rules.
Best of luck.