"Obama has been a wonderful President for two reasons. First, he is a handsome guy. Second, and more substantive, he is a wonder commander-in-chief and a brilliant economic policy planner."

Am I right to use "more substantive" rather than "more substantively" since it describes the notion that Obama is a wonder commander-in-chief and a brilliant economy policy planner?

My understanding is that I should use an adjectival phrase to describe a noun and an adverbial phrase to describe a verb. I am not sure if this is relevant, but I remember seeing people use "first/second" and "firstly/secondly" interchangeably.

Thank you.
You can use first/second/third or firstly/secondly/thirdly.

Just take care not to mix them up.

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Dear Rover,

Thank you for your response.

What about my question about substantive or substantively? Thank you.
I prefer substantively.