I believe it's all a flat rate; I would increase his attack non-the less.

I would recommend enhancing life. Most players will aim your guy, and his health is severely lacking even at 4*.

Speaking of Guy, how much of a boost does he provide? He doesn't list a % or anything.

:@;P:@;P:@:@:@:(:(:o:(:o:(

This is one that has been bugging me for a while too but can't find anyone that knows for certain and I can't spear the ridiculous amount of meat* that it would take to test it. Like mentioned above there's no % on the description which could mean nothing but could also mean that the increase is relevant to the character's attribute (ninjutsu) level, after all not every clone summoning description says that their strength is related to the caster's attributes but they are.

http://*.qq.com/tool/huoying/mnq.htm

The team simulator tells you there. Just mouse over, you'll see the % for everything most of the time.

I'd say life too. With life, he can prolong his presence on the battlefield. And by longer presence = longer buff. Presence = buff.

Depends what you want from him, mine is for 9 tails mainly so +atk was obvious choice.