Ple-Sen
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I would pick either Olivia, as you planned, or Lucy. Both have great weapons, which are always useful. However, it's still unknown what Olivia's sword will give at full upgrade. Lucifer's spear is useful both in the grid and in the protagonist's hands. In terms of strength, both characters are also quite useful, although the Primarch looks better here. He is probably the strongest character in his element.