Cinderpelt x Fireheart
CinderFire / FireCinder
CinderFire was once considered non-canon by fans, and the brief spat between Cinderpelt and Snadstorm was merely Cinderpelt being a protective friend. In fact, before clarified by a collection of evidence and word from Victoria Holmes, their relationship was seen as purely that of a close friendship between apprentice and mentor.
However, a closer look at certain behaviors displayed by Cinderpelt reveals that she did have an interest in Fireheart that as a medicine cat she was forbidden to pursue, and that he did not return. Even so their age difference implied to many fans that it was just a simple crush. However, it's brutally apparent in Sunset that Cinderpelt's feelings are much more then a crush, and she carries them to the grave.
THEORIES AND FAN VIEW
Most fans reach a consensus that Fireheart did not return Cinderpelt's feelings, nor was he at all aware of them. Though it is mentioned in a brief page that being around Cinderpelt does make Fireheart happy, his feelings are already occupied with Sandstorm and Spottedleaf, and he views Cinderpelt as no more then a good friend and former apprentice.
"Both she-cats were being very polite to each other, but somehow Fireheart felt it wouldn't take much for them to unsheathe their claws. He wondered why, then decided he didn't have time to worry about petty squabbles. 'We'll go and eat, then,' he meowed.
'And afterward you need to rest,' Sandstorm told him. 'You've been on your paws since dawn.' She gave him a nudge, propelling him toward the main clearing.
Before he had taken more than a couple of paces, Cinderpelt called after him, 'Send some fresh-kill for me and Speckledtail. If you've got the time, that is.'
'Of course I've got time.' Fireheart felt completely baffled by the tension that had chilled the air. 'I'll see to it right away.'
'Good.' Cinderpelt gave him a curt nod, and Fireheart felt her blue gaze trained on his back all the way across the clearing."
A Dangerous Path, page 104
This conversation between Cinderpelt, Sandstorm, and Fireheart gives away a lot early in the books on the state of her affections toward him. Although it was Cinderpelt who pointed out to Fireheart that Sandstorm cared for him very much, she lets herself act with open iciness towards Sandstorm, revealing her jealousy.
Being Sandstorm, she reacts in the same way, but Fireheart (being Fireheart) is again totally oblivious to anything in the way of a she-cat's feelings towards him. The line about Cinderpelt's eyes following him also indicates obvious remorse and bitterness.
"Every word she spoke tore into Leafpool like a badger's fangs. Fury surged inside her. 'You can't possibly understand!' She spat. 'You've never been in love!' Cinderpelt's blue gaze rested on her, unspoken thoughts flickering like minnows in her eyes. 'Its easy for you,' Leafpool went on bitterly. 'You've never wanted anything else.'
The medicine cat flexed her claws, and her neck fur began to rise. 'How do you know what I want?' There was a hint of a snarl in her voice. 'How do you know what hopes I gave up to follow the path StarClan laid down for me?''"
Twilight, Pages 246 - 247
This quote was particularly revealing, and shocked both the fans and Leafpool. As Leafpool accuses her mentor of not being able to understand, Cinderpelt replies just as harshly that Leafpool has no idea what she wanted. Now, this could be Cinderpelt talking about her hopes of becoming a warrior before her accident, but the way in which she replies to Leafpool's statement (concerning "You've never been in love!") and Vicky's confirmation make it seem as if she is actually talking about her longing for Firestar.
"Fireheart met her laughing eyes and felt himself relaxing. As usual, being with Cinderpaw was lifting his spirits. He would stop worrying about Cloudkit and get on with the job they had come to do."
Forest Of Secrets, page 84
This does not exactly suffice as "evidence", but perhaps more of a subtext (or perhaps just a cute fan thing). Though he canonnly has no romantic feelings towards it, it should be noted that Cinderpelt and Fireheart would make a good couple in the way they get along, and that being with her makes Fireheart happy.