What Luksy said. They were definitely together. Also, in the church, she laughs because Cloud's mannerisms and life seem to be uncannily like that of Zax's. It's a clever use of foreshadowing by the writers. Really subtle. Cloud has mimicked his life on Zax's - and she starts laughing that he seems so similar. This is also further established in the park.
Gongaga:
Gongaga
------------------------------
{AERIS}
“I had no idea Zax
grew up in this village.”
------------------------------
{CLOUD}
“Somebody you know?”
------------------------------
{AERIS}
“I told you,remember?
My first love.”
------------------------------
{CLOUD}
“…”
------------------------------
{AERIS}
“Zax…
Soldier,first class.
The same as you,{CLOUD}.”
------------------------------
{CLOUD}
“Strange. There aren’t that
many who make first class,
and I’ve never heard of him.”
------------------------------
{AERIS}
“That’s all right,it was a long
time ago. I’m worried because
he disappeared.”
------------------------------
{CLOUD}
“Disappeared?”
------------------------------
{AERIS}
“Must be five years ago now.
He went to work and never
came back.”
------------------------------
{AERIS}
“Zax is a real lady’s man.
He probably met somebody else.”
------------------------------
{AERIS}
“Hey,what’s wrong?”
{CHOICE}I’d be worried too,I guess
{CHOICE}(What a slimy…)
------------------------------
{AERIS}
“It’s okay.
But he should at least
contact his parents.”
------------------------------
{AERIS}
“Are you… jealous?
Hmm? Hmmm?
Are you,{CLOUD}?”
------------------------------
{AERIS}
“Just kidding. Sorry.”
------------------------------
{AERIS}
“I’d kinda like it
if you were though.”
Even if "first love" isn't correct here [Luksy?], this is still pretty good evidence.