Due to his age, Nephylite was a man of principles and hence chose to accept Robin's apology and the gift he proposed to him.
A gift that would help Alfin get rid of the poison coursing through his veins, once and for all.
But unlike Robin, Nephylite was well aware of what this small flower was capable of and the danger it posed to higher ranked beings.
Like Lakyus has said, this was truly poison to the strong which when used properly would render them weak and powerless.
Which is why Nephylite accepted the deal and took it with him.
Through this, he not only obtained this cursed flower, but also became aware that it still existed and people in UTENA might be aware of its source. Which he needs to deal with at the fastest.
But for now, he focused on something important. Which was taking Alfin to a safe place where none can harm him ever.
And there was only one place in his mind like that.
