Typically the Marketing VP or Manager or Director (senior marketer) is responsible for the marketing plan, although she/he also has input from other departments such as sales, operations, finance, product development. In a small business, one person can be responsible for developing the marketing plan.

All businesses need a marketing plan: it is the foundation for sales (without marketing planning, sales can be a scattered effort).

Yes an Investment Company needs a marketing plan: what's the market segmentation, target market, product differentiation, product life-cycle, promotion plan, pricing strategy, distribution program and so much more.