Spoilers beware!

Evident between playable characters in their videogames:

• Arche x Chester
• Stahn x Rutee
• Keele x Meredy
• Kyle x Reala
• Natalia x Asch
• Tear x Luke
• Marta x Emil
• Jude x Milla

Unsure or one-sided between playable characters:

• Farah x Reid
• Loni x Nanaly
• Genis x Presea
• Shirley x Senel
• Chloe x Senel
• Hubert x Pascal
• Leia x Jude

With non-playable characters:

• Klarth x Miranda
• Leon x Marian
• Regal x Alicia
• Senel x Stella
• Presa x Alvin

Suggested in official material:

• Kongman x Philia
• Colette x Lloyd
• Sheena x Zelos
• Mikleo x Sorey

For teh lulz:

• Rutee x money
• Zelos x every nice body in the worlds
• Yuan x Kratos
• Rita x Estelle
• Richter x Aster
• Pascal x Sophie