Question: あとな、わしの名はバルタザールだ。

Is it "And my name is Balthazar" or "My last name is Balthazar"?

Answer: 
「**あとな**」 is a light and informal way of saying "**in addition**" and it has the same kind of nuance as "**Oh, before I forget,** ~~".

Think of 「な」 as a variant of 「ね」, the sentence-ending particle.

You can say just 「あと」 without a sentence-ender in informal speech. Or rather, I should say that 「あと」 is not used in formal speech. In formal speech, you would use 「**それから**」.

Seems to me like someone had mispronounced バルタザール's name and バルタザール is now correcting it, but if that is not the case, please forget this comment.
Edit: l'électeur is more correct on this one. It really sounds like he is correcting the other person. I didn't think about that.

It means "And, my name is Balthazar"/"Also, my name is Balthazar".

You can find the exact definition of 「あと」 [here](http://jisho.org/word/%E5%BE%8C) (the 7th one). It is not written in kanji, because in novels the writers often chose to write some common kanjis in hiragana in order to not convolute the text (if wanted, most things in Japanese can be written in kanji).

「な」 is one of the colloquial endings (I'm unsure how this is called in English) such as 「よ」,「さ」etc. 「な」 is exclusively used by males and in this context it sounds rowdy. I don't know the novel you are reading, but Balthazar seems to be a self-confident warrior/bandit type.

If the writer wanted the sentence to mean "My last name is Balthazar", he'd have written 「わしの名字はバルタザールだ」 or some form of that.