I'd say it's good. The manga has a complex and engaging storyline that keeps you hooked. The art style is also distinctive and adds to the appeal. But be prepared for some intense and emotional moments.
Lucy does survive. The manga's ending provides a resolution that leaves her with a chance at a new beginning. There are various factors and plot developments that contribute to her survival.
Definitely not a happy ending. The manga takes some dark turns and doesn't wrap up in a traditional cheerful way. It's more thought-provoking and emotionally charged than just happy.
One way is to create a common threat. For example, a new powerful enemy that neither Naruto's ninja world nor Lucy's world can handle alone. This forces them to team up. Another way is through a portal or some kind of magical/scientific accident that brings them together. Then, the story can unfold as they learn about each other's powers and worlds.
Lucy is a very important character. She is a Diclonius with a tragic past. Her powers are both a curse and a means of survival. Kouta is also important. He has a connection to Lucy's past and his kindness towards her, despite her nature, is a key part of the story. Nana, another Diclonius, plays a significant role as well. She has her own journey of self - discovery and her relationship with the other characters is complex.
One of the main themes is the exploration of human nature. The story delves into the darker aspects of humanity, such as prejudice and fear. Another theme is isolation. The characters, especially the Diclonius like Lucy, are often isolated due to their differences. It also touches on the theme of redemption. Some characters strive to overcome their past mistakes and find a way to be redeemed in the eyes of others and themselves.