there was only friendship between these two people. Tuomas wrote songs of his experiences in love, life. so he didn't actually wrote like "Feel for You" or "Ever Dream" to Tarja. he wrote it to someone who was in his life at that moment. he has never introduced his girlfriends to media and I think it's OK since he has his own personal life of course. just read the Nightwish-book. you can find many answers there.
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Let me fly, let me be free. To stay beyond this life, run across the sky. Let me see the sun again