Advice..will adding a second cat change my cats personality?

How long have you had your cat? If its only a few months then adding another should not be too much of a problem - also the great members of this site will be able to give you valuble advice on the best way to introduce a new cat into your household.
I will go against the grain of most people on this site though, and advise against it. At the moment, your cat is the centre of your world - and he knows it. Inroduce another and of course things will change, including his personality.
Years ago i had a much beloved cat and introduced a second. Although they tollerated each other, played and sometimes slept near each other. I felt that my first cat hated the intrusion and was never the same. The first cat was always "top dog" but even so, i knew that he prefered it when he was on his own.
The second cat was a bit of a hobo and i was always having to go and collect him from neighbours where he had "moved in " (telling me something maybe?) The first cat LOVED it when he was away and went back to how he used to be.
Unless i got siblings, or cats that grew up together, i would never introduce a "new" one again. I have one very happy boy at the moment and thats how i want to keep him! Just because they are not fighting each other does not mean that its not all out war between cats. There is so much we dont know about feline body language.
Of course, this is just my opinion and my experience. As i said, i am pretty certain that i will be the only person to advise against it as most of the people on this site have more than one cat...

Do you have any idea if your kitty has ever lived w/ another, or been around them at all?

If you follow a proper introduction, many times if you take things slowly they integrate just fine - and in awhile you have best buds.

When a new cat arrives in the household, there is often a change in the existing cat's purrsonality. Their territory has been 'invaded' in their minds, is a way to think of it.

Here is an article on how to introduce cats. Many people make the mistake of bringing the new one in & when they don't immediately get along they give up hope & seek a new home for one of the cats.

What it comes down to is that every cat is different & every 'group' of cats gets along differently. I've had some cats who can be introduced/integrated in a week & get along fine, yet I've had others who I spent a month integrating & even now their relationship is tenuous with other kitties.

Expect a hassle when first introducing the cats, and your kitty isn't as likely to be friendly and may be on guard. Its the 'stranger danger' effect.

They can become best buddies though assuming they were socialized with other cats, and that can be really rewarding.

That way when you are gone they have mental stimulation and someone to chase and wrestle with for exercise. And nothing is cuter when they are all cuddled up together and grooming and playing w/ each other. Regarding affection to yourself, with my two they still both expect attention and playtime, follow me like a shadow, and sleep in the same room as I'm in. You're still the bringer of food, play toys, grooming brushes, and other goodness after all which doesn't change.

Other cats, especially older ones who have always been alone, will merely tolerate an intruder or worse and just find the stranger stressful though, so it depends on the kitties.